A big "No" to your question. If you are able to interact properly with the niche-based forum sites, it will benefit you a lot. Sometimes around there just visit the forum sites only for creating the links, which is totally a wrong way. Well, you should try to solve the problems of others and yours. Then the backlink you will get will be really impressive!
Long story short, adding a forum signature won't really hurt your SEO if practised properly.