There is some truth to this, but here is the story. Acid
fixing baths can retard washing for two reasons, both of
which are related to the pH. First, the pH of the gelatin
decides the electrical charge of the molecules which in turn
contolls the degree to which thiosulfate ions and ions of
the reaction products are attracted to those molecules.
Gelatin is peculiar because it is neither an acid or base
but has characteristics of both. Such substances are known
as amphoretic. Gelatin will take on the pH of the last
solution it is treated with. However, gelatin has a
characteristic which _is_ pH related and that is the pH at
which the charges in it are at equilibrium. This is called
the "isoelectric" point. The isoelectric point is also the
point where the swelling of the gelatin when wet is minimum.
For most photograhic gelatin the isoelectric point is
slightly on the acid side of neutral. The isoelectric point
is a function of the method by which the gelatin is refined.
  When the pH is below, that is on the acid side, of the
isoelectric point, the charges are such that they tend to
attract the thiosulfate ions. This is due to the familiar
rule that opposing charges attract and similar charges
repel. When the gelatin is brought to the alkaline side of
the isoelectric point it will repel the thiosulfate ions and
so they will wash out faster. Because photographic gelatin
has an isoelectric point which is very slightly acid making
it neutral in pH is sufficient to make the charges favorable
to washing.
  The second reason acid fixers tend to retard fixing
applies only to fixers containing Potassium aluminum
sulfate, AKA White Alum, as a hardening agent. This
substance hardens gelatin only over narrow range of pH which
is somewhat on the acid side. The hardening action is
destroyed if the emulsion is subsequently made alkaline. The
alum also binds other substances to the gelatin molecules.
This effect is described as being similar to the mordanting
used to make dyes fast. At neutral pH the mordanting action
no longer takes place but most of the hardening action is
still effective. If the gelatin is made more alkaline, for
instance by treating it in a 2% solution of Sodium carbonate
or Sodium Metaborate (Kodalk) both the mordanting action and
the hardening are destroyed.
  Hardeners are used in the fixing bath to prevent undue
swelling of the emulsion in both fixing bath and subsequent
washing. The gelatin is quite vulnerable to scratching and
gouging when swollen. Newer films have emulsions which are
thoroughly hardened in manufacture and probably do not need
the further hardening of the fixing bath but some still do.
  The effect of these two binding forces on the thiosulfate
ions and on the fixer reaction products (silver and
thiosulfate complexes) is to extend washing times by perhaps
three times.
  The effect of both binding actions can be eliminated if:
1, the gelatin is never made acid; 2, if it is not treated
with alum hardener and left in an acid condition. A neutral
or somewhat alkaline fixing bath accomplishes the first
purpose and, indirectly, the second because an alum, or
other metallic hardener, will not work in an alkaline
environment.
  What is the disadvantage of a neutral or alkaline fixing
bath? Mainly that any carried over developer remains active
until either washed out or exhausted or enough halide is
dissolved to prent further development. Secondly, an acid
stop bath can not be used and in any case any developer in
the emulsion would be reactivated in an alkaline fixer so it
would do no good. If the fixing bath contains sulfite, which
it must to preserve the thiosulfate, the developer will not
produce stains due to oxidation products but it may make a
difference in the final amount of development of the
negatives. Usually in non-acid development systems, like
some color films,  an intermediate wash is used between
development and fixation which is sufficient to wash out the
bulk of the developer. The development time and this rinse
time are calculated to result in the proper amount of
development. This same thing can be done in black and white
of course. Non acid processing is used for some color films
becaue the dyes which make up the image are bleached or
otherwise changed by the acid.
  What are the advantages of acidified fixer? First,
development can be stopped at a definite point by using an
acid stop bath and any carried over developer will not be
reactivated in the fixing bath. However, the main reason for
using acid fixers is to make possible the use of metallic
hardening agents. Hardening was once absolutely necessary
but has not been for some decades.
  Now, the saving grace is that the advantages of an
alkaline or neutral fixer can be obtained even when using an
acid, hardening, fixing bath, by the use of a correctly
compounded Sodium sulfite wash aid. Kodak Hypo Clearing
Agent is such a wash aid, and is the only one on the market
AFAIK, which is compounded correctly to get the full
advantage of adjustment of the emulsion to the critical pH
where it is above the isoelectric point and above the point
where alum hardener acts as a mordant but still at a pH
where hardening is preserved. This is done by buffering the
Sulfite to neutral pH with bisulfite. However, most of the
ability of a sulfite wash aid to accelerate washing is due
to a different mechanism, namely an ion exchange function of
the sulfite itself. Sulfite tends to actively displace
thiosulfate and silver complex ions from the emulsion. The
sulfite ions are left in their places but are much more
easily washed out. The use of a buffered Sulfite wash aid on
film reduces the wash time by about a factor of six over
that when a hardening acid fixer is used, and about three
times the wash time of a non hardening fixer. Becuse of the
ion exchange property wash time is even less than when a
non-hardening neutral or alkaline fixer is used, perhaps by
half. This ion exchange effect also takes place in the
fibers of uncoated paper support (non RC printing paper) as
well as in the emulsion. While the paper support does not
wash out by a strictly diffusion process, as does the
emulsion, the rate at which thiosulfate bound up in the
fibers leaves during washing is still very considerably
accelerated and wash times shortened to about one third of
untreated prints.
  Buffering to neutral has two advantages over using a
simple sulfite solution: it preserves hardening where that
is desired; it places the gelatin close enough to its
isoelectric point to minimise swelling thus resulting in the
shortest diffusion path for the various ions which is its
desired to wash out.
  Sulfite wash aid was developed from research into sea
water washing. It has been known for over a century that sea
water washes out film and paper considerably. However, the
reason for this was not known until research into it was
carried out during and after WW-2. Sea water washing was
much used in practice during the war in locations where
fresh water was at a premium such as on board ships at sea.
Sea water washing requies a final rinse in fresh water to
wash out the halides and other substances in the sea water
which other wise will cause very rapid degration of the
image. But, even with this rinse, there is a very
consdierable saving in fresh water.
  Kodak Research Labs discovered that the salt most similar
to sea water in accelerating washing was Sodium sulfite.
Other sulfite salts work but not as well. some othe salts
have accelerating effects but are well below sulfite in
effectiveness. Kodak Labs also determined that the pH should
be neutral, for the reasons mentioned.
  In addition to the Sulfite and bisulfite KHCA also
contains two sequestering agents, EDTA Tetra sodium salt,
and Sodium citrate, which bind up carbonates and prevent
them from being deposited on the surface of the emulsion.
This is mainly necessary when there is no rinse between an
hardening fixing bath and the wash aid or when the wash aid
is re-used.
  Sulfite wash aid also makes some otherwise insoluble
fixer reaction products, or those tightly bound to the image
silver, soluble. The effect of this is to extend fixer
capacity because these products are left by partially
exhausted fixing baths. Nonetheless, for good premanence
fresh fixer should be used or a two bath fixer should be
used.
  The point of all this is that fixer does not work any
better when alkaline. The effectiveness of thoiosulfate
(either sodium or ammonium) is not affected by the pH of the
solution: it works equally well in acid, neutral, or
alkaline solution. The only justification for a neutral
fixer in B&W phtography might be the preservation of the
stain image when using a Pyro developer, but I am not
convinced that what people are seeing is not an increase in
the overall stain (fog level) when doing this. The imagewise
stain left by Pyro is NOT a dye but rather a quite tough
pigment related to humic acid. This stain image, BTW, is
probably more permanent than the silver image it
accompanies.

 The ph of ALL off-the-shelf fixers is below seven. The ph of
ALL off-the-shelf stop baths is below seven; the shelfs of the
usuall photographic supply houses.
 If you found that the claim is true and wished to purchase
an alkaline fix where would you turn?                      Dan