To be clear, I received the cel in this conditon 
point one...ah hum...cent one. I think that it is 
great signed or unsigned.

My other cent is this: 

How many artists "sign" their work, or 
representations of their "art"? 

answer:
 Almost all of them, given the opportunity.

To that end, to assume that there is less value 
assigned to the acetate or that is some way it is 
less asthetically "correct"_TO ME_ is absurd, if 
the creator or major person involved in the 
development of the anime applies his 
stamp/signature of appreciation, and/or 
recognition, (to quite possibly a fan that has 
waited for hours in a line at some AnimeCon) in 
fact, this in itself being a rare and valuable 
opportunity, could suggest an increase in the 
value of the cel quite on the contrary.

We are not dealing in Piccaso's here! But on the 
other hand...
Wouldn't you like a signed Piccaso? Would you 
prefer Piccaso not sign his paintings?

Of course this is an extreme one-cent analogy ^-^ 
of course...I'll stop there...cause I am heading 
towards nickels and dimes with this..
All I am trying to do is offer a cel.....
take it or leave it.