LOL!
Who says
SEO types don't like cool t-shirts?
Staying "relevant" certainly isn't going to hurt. Remember, it's your posts that count IMO. Indexable content that searchers with the same questions, looking for the answers, trying to find information, search for and find. Every post you make on a forum gives the searcher a "reason" to follow the the links you provide based upon
what you post. Signature or otherwise. Gives the searcher an opportunity to not only follow the links, but an opportunity to tell friends or even link to you.
Ever follow a signature link because you like what the person had to say or ask?
Dave