[ ... ]

    I think that the initial article in this thread mentioned the
Nikon D70 (or was it the D70s?) and the 18-70mm "kit" lens.  I've been
using one for a bit over a year, mostly with the "28-105mm f3.5-4.5 D"
lens, with the hood designed for it.

    However, I just got (last Monday) the normal "kit" lens, the
"AF-S 18-70mm f3.5-4.5 DX".  This was bought separately, so I don't know
whether a lens hood is supplied with the camera kit, or whether you
bought your own separately.

    The hood supplied with the lens is a "petal" style, and the
Nikon model number is HB-32.  It does bayonet on as described somewhere
else in this thread, and I can get no vignetting from the lens hood at
*any* focal length within its range.

    Note that it should be put on properly, which involves sliding
it onto the lens with the symbol:
          +----------
          |
          0

(the '0' above should be a solid dot) in line with the white dot at the
front top of the lens, and then rotated until the symbol:

       ----0

(this time the '0' is hollow) rotated until it is in line with the white
dot mentioned above.  You have to overcome some resistance to go from
the left-hand end of the "----" to the "0", and there will be a
noticeable "click" when it does.

    If I rotate the hood so it is not in that locked position, I do
see some vignetting, so that is what makes me think that you don't have
it locked properly (if you are indeed using the HB-32).

    Note also that you can store the hood on the lens by mounting it
backwards, in which case you need to interchange the two symbols from
start of installation to locked position.  This has the advantage that
the hood does not add to the length of the lens when it is in your
gadget bag.  My 28-105mm mentioned above also has a reversible hood, and
find it easy to use the lens with the hood stored in the reverse mode,
as it allows easy access to the zoom ring -- but not to the manual focus
ring.

    Unfortunately, the reversed hood on the 18-70mm blocks access
to the zoom ring, while still allowing access to the manual focus ring,
which is less often used than the zoom ring for most shots.

    You initially asked about when to use the hood, and I have
already seen good answers to that part of the question, but a secondary
one which you did not ask is when to *not* use it.  One example of when
not to use it is when using the built-in flash and the wider zoom
settings of the lens.  The hood casts a noticeable shadow on the bottom
center third of the image.  (IIRC, there is even some shadowing without
the hood at the 18mm setting.)  If you use the SB-600 or the SB-800
flash (even on the camera), you should not have this problem, though I
have not really tested this yet.

    So -- I store the hood on this lens (when on the camera) in the
normal position, to allow easy access to the zoom ring.  The new style
of Nikon lens cap supplied with the lens allows easy reach inside the
lens hood to remove or replace the lens cap.  The cap is tethered to the
lens with a short string attached to an elastic band around the lens,
and to the lens cap with a plastic button with double-stick foam tape.

    BTW -- I also recommend making a tether to go from the camera
strap mount to the plastic protective cover for the LCD display, as the
cover tends to hook on belts or other objects and get lost.  I lost my
first one precisely one year ago tomorrow, and made the tether as soon
as I got the replacement.

    Enjoy,
        DoN.