Well,
I love my signed cels...but they're signed by 
the original manga artist.  Im not sure how Id 
feel having a voice actor/tress sign a 
cel..depends on who they were.  I think if it 
was the original Japanese voice, Id love it.  I 
am not one to ask I guess...since all my stuff 
is Watase related...  Id have her sign 
everything in my house, hahahah!  Also what some 
people do, is they put a spare piece of acetate 
over the cel and have the artist/voicer sign on 
that.  That way it's removable later if you dont 
want it.