Forum Moderators: martinibuster
Many people do that, but what the masses do isn't necessarily the best thing to do. I rarely ever see anyone with forum sigs in their backlinks in the top ten. The few times that I have seen that, however, it was because they also had a lot of legitimate links. I don't think the forum sigs helped, if at all.
Another thing to consider is that one link from one site is the limit of what is going to help. More links from a site may increase the likelihood that your page will get spidered, but one link is often enough. So spending your time in a forum dropping forum sigs would end up a waste of time.
For a website I care about, I would not want forum sigs in my backlinks.