yes it help, But quality is important for ranking. But we have to keep in mind, all those low quality links add valuve end of the day. All depend on time and money.
Basically a free low rated or low quality link usually, many forums are do follow so at least it counts. Plus you can help someone, ask a question, etc and get a backlink all in one pop so for many it's win win situation.
it helps but very little, so in order to increase the strenght of the links you will have to have houndreds of them
If the signature link is dofollow, it will help in serp. If the signature link is nofollow, then it is mainly for targeted traffics only. Btw, try to get more quality backlinks from different domains and ips. Make sure the post is unique too.
Internal links and Backlinks are much important for getting high PR. So I thing Forum posting is also good..!
I think forum posting will help you in many ways like gaining back links and ofcourse brand of yours. lets say if you are answering threads as expert and then it would be helpful to promote your brand. one more thing if you are posting in same theme forums then dofollow signature links will also helpful..
Forum posting is one of the best ways to get Back links , this may helps to get good position in SERP that will give some good traffic if u target good keyword. But only forum posting not work.. Google consider many factor to rank a site
Essentially, it helps to a small extent and that's where it ends. Multiple links over the same Class C IP do not help you in ranking in the SERP's. So if you told me you were planning to post across 15-20 different do-follow forums then I would believe it would have some benefit in the ranking. The goal is not to post useless content regardless, you should be adding value to the forum and trust me you'll be rewarded.
Forum posts are good for relevancy, themed anchor text, and easy deep links; not to mention direct traffic if you find a busy community targeted at your audience.