I don't have a clear cut answer for this question, so I didn't vote, but I do have strong opinions on this topic. The answer is: it depends.
BACKGROUND INFO
I bring shikishi or limited edition posters to be signed. Also, most companies provide nice printed shikishi at all the major cons for their celebrities.
However, I asked Kawamoto sign one of my top Faye cels. I usually would not have cels signed, but from observations of other autographs he has done, I know that he is really careful about where he signs it (he spent 4 minutes deciding where to sign on the key cel that I handed him), so it does not detract from the art. He was the character designer, so like someone has mentioned, it is like an artist signing his own work. And who is not a fan or his body of work?
I didn't have an extra layer for the autograph, and now I kind of regret it because I've heard how the ink might degrade the cel. So far, it doesn't look like it's eatin' it away or anything though.
It does bring a special meaning to me personally because I can relive the memories of that particular con and how excited I was to meet one of my idols when I look at that cel.
At last count, I think I have two other signed cels, so I guess I'm leaning to the "no sig" side.
ON PURCHASING SIGNED CELS
I don't agree with mangaka signing cels unless they were included in the anime staff -- I would bring a manga artbook, a manga art poster (no bootlegs), or the original manga for them to sign (yes, I'm picky). However, I would consider buying a cel signed by them, because mangaka autographs are harder to get.

I have noticed on YJ that people usually get a lower-quality cel signed by popular Japanese talent, which then raises the value of the cel. Unless the Japanese talent is one of my all-time favorites (i.e. Megumi Hayashibara, Nobuteru Yuuki, etc.), I wouldn't go for it.
I would never buy a cel with an autograph by the American dub VA, nor let American dub VAs sign a cel. They didn't originate the role, was not part of the original production process in Japan, etc. I think it actually decreases the value of the cel immensely and zeroes out all appeal, but this is because I personally think the majority of dub actors are terrible.
