I've never been afraid to use a pencil for notations on the back of the
print. Quite often, I write exposure / contrast / dodge&burn settings on the
back of the print before it is exposed and processed. Pencil doesn't migrate
when damp. Be sure to write lightly so that you don't emboss the paper
through to the front - using a hard, smooth writing surface helps prevent
that.

Because of the surface coating on RC papers, you'll find it difficult to
write anything in pencil. Some RC papers have a coatings on the back that
take pencil better than others, but never as good as non-RC papers. On the
other hand, the plastic coating will resist ink bleed-through better than
fiber paper should you choose ink. I buy special archival pigment pens from
a local art store for writing on prints in ink when I need to. I NEVER use
commonly available Sharpie markers though - I've seen too many of them bleed
over time on many different surfaces including plastics.

If I use ink on any paper, I try to contain all notes to the white margin
area so that potential bleed-through won't affect anything in the picture
area.

Bill Schneider

For RC paper, I use a china marker.
For fibre paper, I use a #3B or #4B pencil. You do not make sharp points
with such soft pencils, and you do not need to press as hard as for an HB
(#2) pencil.

Writing on the back of RC, I use a really soft pencil and make sure that I
write on a hard surface to minimize the effect of press through to the
surface of the print.  Two pencils I have right now in the darkroom that
work well are a Sanford Design Drawing 3800 6B and a Prang Drawing 3B.  Some
very soft pencils have hard bits in them, (perhaps from the clay the
graphite is mixed with?)  but these two brands give good results on a tricky
surface.  I have also had better luck with drawing pencils than with
drafting pencils.  And with wood wrapped pencils vs. drawing or drafting
leads. I usually take a scrap of RC paper along to test before I buy.  You
don't want a really sharp point either.  If you use a sharpener blunt down
the sharp point a bit on a piece of paper or even sandpaper before using on
prints.

Ball point works and I like the medium Parker ballpoint in black, seems to
skip less on most surfaces than cheaper pens (haven't tried the new gel
refill suspect it would be less satisfactory), but I only use ball point on
work prints, ditto for Sharpies, etc.  Print collectors and museum types
throw up their hands in horror when they see ink on prints, us mere mortals
don't sweat that kind of thing, but who am I to frustrate people who will be
collecting my prints in 150 years!  On the other hand, my complaint
department will have been closed for a long time by then.