You're quite right about it being a personal
thing.  :)  And from my personal point of view,
having a cel signed completely destroys its resale
value, at least if I'm considered the target market.

Now, if you're thinking of KEEPING the cel, it's
obviously up to your own aesthetics whether you'd
like to have it signed or not.  But I'd like to
suggest an alternative, in case you think you ever
will need to sell.

Most art supply stores sell clear acrylic sheets,
the same material used for cels.  If I was
planning to have a cel autographed, I'd pick up a
sheet of this, cut it down to the same size as the
cel, and have THAT signed.  Since it's
transparent, you can still see the cel underneath
the autograph -- you've basically just added
another layer.  And you can remove that layer if
you want to sell and the buyer doesn't like the
idea of a scribbled-on cel.

Hope this helps!