Yeah, that is pretty much how I do it, though I have limited the height with max-height. I don't care much about the content they put it in, but I hate it when the height of the signature is heigher than the actual message.
I was thinking some more and if you actually strip bbcode from the signature, you remove url's as well, so it actually would require an addon, or do the dirty one and remove the WYSIWYG editor from the signature box...