As you said, this is very much a matter of 
personal preference. ^_^  Myself, I don't care 
for signatures on cels.  I don't own any that 
are signed, have no desire to have any of mine 
signed, and would never pay more for a cel 
simply because it had a signature.  If presented 
with two near-identical cels, one signed and one 
not, I'd take the unsigned one.

That said, if you do want a cel signed, I like 
Evilminion's suggestion of having the signature 
on another layer. Of course, the owner of a cel 
is free to do whatever s/he wants with it! ^_^

-Pasiphae