I think there should be a preview feature so you can see what your signature will look like before you apply it. I get irritated on posting it without knowing exactly what it's going to look like. Or having to preview it by typing it in a comment box somewhere first.
Sure it's not a super-awesome feature, but a helpful one.
I don't ever use signatures, but if I ever need preview something, I usually pop open a forum page in a new tab, paste my text into the textbox on a thread, and preview from there. Then make any changes, close the tab (without sending the comment) and I know what my text will look like.
I do that too, but a preview would make things so much easier... They might as well add it, what could it hurt? There's no downside to it what-so-ever.
Sure it's not a super-awesome feature, but a helpful one.