FranticFish - 10:17 am on Feb 19, 2010 (gmt 0)
The OP's question was "is it ok if I build forum links by participating in several niche forums related to my area".
Depending on the niche I've seen forum links helping. A site I saw recently was just outside the top ten for a nice local term with nothing but forum and other social profile links related to their niche. Not that many, either, and only sig links. They didn't have a k/w domain or an optimised site.
So, in the mix, I think they can be useful - depending on the niche.