I believe that if the cel is cheap, then a sig 
might increase the value. but if the cel is 
really something, then to a lot of collectors it 
might decrease the *monetary* value. for me, I do 
not like signatures on my cels, and last year I 
bought an FY cel with watase's signature. when it 
arrived, I was thrilled to discover the signature 
was on a separate cel layer(smart cookie!). so 
I've kept it on there, knowing that I can easily 
detach it.

/Mina