I also don't care for signed cels personally. If 
I were to meet an artist I was really fond of, a 
sketch might be night, or a signature on a blank 
piece of acetate, but never on the cel iteself.

Of course I am sure this differs from person to 
person. At one con I did buy a cel at the auction 
and it was signed by the character designer who 
was a guest, but that was more for sentimental 
reasons as it was a my first con, and less about 
the cel itself.

However I am probably not the right person to 
ask. Everyone has different reasons for 
collecting and deciding what appeals to them. To 
most fans an original signed sketch of a favorite 
character is the ultimate treasure, however it's 
not the ultimate for me compared to a rare cel. I 
don't even have any real interest in them, unless 
of course the artist made it especially for you.

To each their own... if you worry to much about 
potential resale value you will forget to enjoy 
the hobby. Do whatever looks and feels right to 
you. :)

~Kary