Ever since I starting cel collecting, I strive to get some of my cels signed! I actually had a chance to at AX2000 when I met the creator and character desginer of 'Shoujo Kakumri Utena.' But at that time, I wasn't into cels, so I got a personal sketch and autograph instead.
But yes, if it's the creator of the anime, then Iwould love an autograph on the cel. If it's anyone else, VA, character designer, producer, etc, then I would prefer the autograph on a separate layer.
As to the value of the signature on a cel, I think it's up to the buyer whether the added sig is worth more. In most cases, I think it makes a cel more desirable.
Personally, I think English dubbed siggies actually make the value of a cel decrease.
