When I first starting posting fanfic to teh interwebz, I tried to have everything beta read; now, however, I usually just ask a friend to look it over for silly mistakes. Honestly, this is because I've had difficulties finding a reliable, thorough beta reader; most of the betas I've worked with sent my fics back with only one or two quibbles or a "That was great!" tacked on the end.

To be fair, though, this might be because the fanfic site I'm most familiar with (a Star Wars-specific forum) is very, very strict about dishing out concrit. Strict to the point of absurdity, in fact, and so while the site is fun and friendly--and the fanfic is generally quite a bit better than what one would find on ff.n--I've stopped looking for beta readers there.