I think this topic is a very subjective issue and 
of course there's no right or wrong answer. My 
view on signatures is that it's a very personal 
addition to the cel. For example, if I went to a 
convention and got one of my beloved cels signed 
by the director or character designer of the 
series, I would probably cherish that cel for 
life because in that signature, there lies the 
experience of meeting the director or designer in 
person and having him/her sign the cel just for 
me. But on the other hand, if that cel signature 
on the same exact cel was obtained by another 
collector, then I would feel that the signature 
detracts from the cel because I would have no 
personal attachment to it. Anyways, just my two 
cents. :)

Tony