Depends on how many links and from how wide a variety of sites you have other than the links in your sig files.
Say your site is newish. You ad a link in the sig file and all of a sudden have 200 plus links from the one domain. Google is gonna say 'you're kidding right - go to the sandbox, do not collect $200'
If your site has links from 2,000 different sites, to a variety of your pages not just the homepage... then you ad a link to a sig file and all of a sudden have 200 more links from the one domain, google won't have such a big problem with it and you will get a little benefit.
Take into account: Some forums disallow spiders from the sig files. Forum pages are of extremely low pagerank value, despite the pr of the forum home.
Treat forum sigs as advertising and nofollow them until your site has a very well established pool of links first.
Create the equivilent of a presell page in the forum. Usually these types of forums will have a 'review my site' section. SEO a post seeking reviews of your site, then direct the links in your sig file to that presell page.
End result is you have 200 pages of the forum linking to the one presell page within the forum that will appear in your search results... with links to your site... much more effective.