I usually read the initial post, then decide whether or not I'm going to keep reading. If the post interests me, or I think I might have something to contribute, I keep reading. Otherwise, I quit reading and seldom look back. If I decide to keep reading I at least skim the entire thread to see if my opinion or advice has already been voiced, and to see if I'm even still interested in posting.

I think it's just a matter of common courtesy to read the whole thread, so you don't disrupt the flow of the discussion, or repeat what's been said a few, or a dozen, or a hundred times before, and make yourself look like a fool.