Another option for signing cels, is for them to 
sign another sheet of acetate, so when you are 
displaying your cel, you can remove or replace 
the signature at will and have the best of both 
worlds.

Personally, it should not really matter to 
whether it increases or decreases a cel value, 
all that matters is what the cel means to you. If 
you want it signed, get it signed, if not - then 
don't! In the long run, I am aiming to keep my 
cels, so personally to me it does not matter what 
happens to mine. As long as I can keep them in 
the best condition that I can, I'm happy! Welcome 
to cel collecting! ^_^