I've never had a cel signed (I don't think I would either,
however, that's just me), but I would think you should go
with "better safe than sorry". You never know if you'll
later learn that the pen you used somehow degrades acetate,
or does some other nasty thing to the cel. Also, you never
know if for whatever reason you may decide later that you
want to see the cel in it's original state. If the signature
is not on the cel itself, you're still good. Once the
signature *is* on the cel, you're stuck with it there.

Many Sharp Smiles,
--Drac