Geoff Sobering , Jul 03, 2008; 06:49 p.m.
My 20D and 40D both seem to like about +1/3 or +2/3 for "normal" scenes to get a good ETTR histogram. I generally leave both cameras at +1/3 by default.
I use other amounts depending on the subject and lighting. In unusual cases I'll shoot a couple of tests to get the best value for the situation (actually, that's pretty much what I do all the time).