I'd say that the same rules apply with contributing to Forums as they do with creating Website Content.
You should post on forums if the content you provide is relevant and useful, but you should not do it purely for the purpose of creating/generating links.
If the visitors to that forum find the content you provide useful, they may visit, bookmark, share your site to others - creating both traffic and SEO value. Over time, you may become a trusted source of information in an area where you have subject matter expertise, which is very valuable for any company.
If the visitors to that forum do not find your content useful, they will likely ignore you completely - creating very little (if any) traffic and SEO value.