Oh, yes-- it's definitely a matter of personal 
preference. ^_^ Ownership is the key part-- if I 
own something, I can do whatever I want to it, 
for the good or for the bad. And in this case, a 
signature is definitely a matter of personal 
preference. :o)

It's just that in general, if you plan on 
reselling something, it doesn't matter *who* 
signs it-- if it's the original seiyuu, the 
American dub actor, the director, or your little 
brother-- if someone marks on it, the general 
value of the cel diminishes.

But that's not to say that should prohibit anyone 
from doing what they want with their property. 
However, it *is* brought up, just in case there 
are others who are planning to have their cels 
signed. You might think, "Cool! I can get my cel 
signed!" while you're waiting in line, but in 
perhaps three years, you might rather wish you'd 
had them sign the sketch, or something else. :o)

But by all means, it's not to stop people from 
doing what they want with their property. It's 
just giving a heads-up to say that while some 
people may view an autograph as a bonus, others 
may view it as a defacing... and it might come up 
as an issue if you ever need to resell.

-Cres ^_^