It really does depend on what you think of it.  
For example, I have a Sailor Uranus shot signed 
by Ogata Megumi (voice artist) that I would not 
let go of for the world!  But others might think 
it detracts from the cel.

I've actually heard several cel collectors say 
they hate seeing cels signed by people...but then 
there are plenty of others who disagree.  (How 
could they not like it?!  It's Ogata Megumi!!  
*_*)