Have you considered a "test" forum e.g. when I change my signature or avatar, I would like to see what it is going to look like on an actual posting. thanks
This is a better example than I thought it would be. I just changed my signature and it doesn't show in my original posting.
You should be able to use the "Preview" feature to accomplish the same thing. I use it frequently (along with the spell checker) to make sure I've got any BBCode right, etc. Are you sure you're changing your "Signature" settings? It's a separate sub-panel within the Profile and it should show up on the bottom of your post as opposed to what you see on the left-hand side.
I guess the signature either is there or is not in a post, because my previously posted signatures were updated when I added the wave to the end of mine. Seems more friendly, too...