Building profile links on forums for purpose of optimizing site is just wastage of time, if this method has any impact on Seo then I wonder its having very little impact on your ranking .Google can easily detect between natural links and profile links build by you. And I think in case you have lots of profile links with same data and description filled then it might be considered as spam. It’s better not to focus on creating profile page links.
And if you really want to build profile links..
In that case if building forum links is part of your seo strategy then it’s more important to find high quality and High PR sites with absence of nofollow tag in the link pointing to your website. And make sure that you create profile page by filling unique content with unique description about you. And don’t just create profile page with a link back to your site, do participate in forum so that Google can easily index your profile page.