how safe are online forums?

That's up to you. Unless a given forum periodically reviews emails for validity, which I don't recall ever happening, you only give up the capability to be notified of something that the admins in a given forum want to notify you of, or thread subscription notifications, or any other form of communication that would be initiated by other than you.

Again, personally, I have no problem with (and expect) spam directed to my free web account(s) and have never missed anything worth knowing from forums, etc., by using a valid web account for registration. I have a couple of web accounts with one used for low trust forums, harvesting alot of spam, and another used for higher trust uses (not as high as personal ISP account though) which has only the rare spam show up in it.

 

A second web account isn't necessary if you use Mailwasher.