Do you recommend using the rel=nofollow tag on links that are member-submitted, such as from a blog or forum?

My opinion is no. Crap links should be weeded out by human editors / moderators.

But human editors are ... well ... human. Sometimes we don't catch everything or we don't catch it fast enough. And what if a spam member links to a bad neighborhood or lots of off-topic links? What if they do this in their signature and the link is suddenly and diabolically spread throughout your otherwise squeaky clean site? Especially on very large forums, once a thread falls off of page 1, it most likely isn't going to be respidered more than once (or twice if you're lucky).

There's quite a debate about this currently going on. I'm aware of one SEO forum that did it and they quickly lost most of their heavy hitters. I don't think they left because they didn't want to lose the links. I think it was more that they didn't agree with the concept. I do think however that if you don't have the personel to monitor the posts that it's difficult to stop the spammers. So it's your call I guess. One way to do it is to raise the number of post that a member must make before use the sig link. Then if they post junk just to get there you can weed em out before they reach the number. In most of the forums that I frequent, any spam draws attention like flies to a horse barn, and the posters either straighten up or are history. Most forum software gives you the option to not only ban, but to delete all or some of the posts from a particular user.

Speaking of spelling: make that "personnel" not "personel". It's getting late. lol

except for the Signature links rest should be done with rel=nofollow

In my opinion SEs dont even give much weight to links inside posts/signatures any more let alone ban a website because of them.

Not true Get a link within a front page article on a high profile site and then try and say that links within articles don't do much On the contrary, it's these types of links that Google was designed to thrive on ... not little text links in sidebars and footers that are just there because people bought them.

Links from the Forums do count and are worth the efforts to be a member.

Google started to discount bought links or text ads which people buy for their websites.

It now prefers natural links..

Google has always preferred and emphasized natural links. Link buying is just a practice to get around google's algorithm. Google has always taken efforts to counteract it.