Honestly, how many besides myself thought this is 
what this thread was all about?

It doesn't really matter, as from what I can tell 
cels are self-consuming artifacts.  Also scans 
made from them, except for .tif files.  All the 
rest automatically compress and decompress the 
image, slightly degrading it each time you open 
the file.  So in ten years none of us will have 
our originals to look at, either the cels or the 
scans.  What's important is that we look well and 
lovingly at them now as none of them will ever 
look so good again.

But for the record:

Original cel bag: I leave them as they came so 
long as the bag is servicable.

Staples: Leave them: they won't rust unless the 
roof leaks and water drips into my cel books.  
[Sensei checks to see how many shingles blew 
off during Hurricane Isabel.]

Tape: I take it off whenever I can.  If I can't, 
then I remember that it's the kind the yellows 
and dries out, and so it will fall off by itself 
in 2-3 years anyhow.  Corn starch helps get 
sticky residue off.

Dust/Dirt: gently rub with a cotton cloth 
(Kleenex leave as much dust as they get off).

Scanning: out of the bag with the original bg or 
colored mat directly underneath.  Then back in 
the bag separate from the bg or mat.

BGs: bag only if they're the original watercolors.

Cel slightly stuck: slide an old photograph 
gently between the two, image side down.  Look 
especially at the hair tips.  If the cel is 
already damaged ("slightly stuck" often means "I 
already tried to get them apart and can already 
see paint coming off on the sketch in softball-
sized chunks"), leave them together.  If not, see 
how far you get with the old photo.  Take two 
Prozac for every hundred dollars you spent before 
doing so.  

When in doubt, remember that cels, like cherry 
blossoms, should be viewed and loved in the here 
and now, in full knowledge that they won't be 
with us forever.