@numnum, I block some because my main audience is male, but when I blocked "low performing" categories, my eCPM went into the toilet.
Do you use the term "low performing" in the same sense that I would, based on my original post for this thread? If so, then one might conclude that when you block enough habitually low-performing ad categories you'll wind up trashing your CPC so that your total earnings will actually decline despite the fact that you've blocked low performers. Why? It's the advertisers pushing products and services irrelevant to your site's topic who have the deep pockets and hence help bid up the prices for ad space at your site. Am I right about this?
I suppose I'll just have to try category ad-blocking as an experiment, and watch what happens. I'm surprised more Adsense publishers here have not experimented with ad blocking by category in a systematic manner to determine the point at which this sort of blocking becomes counter-productive.