I definitely avoid using the microformat classes as hooks for CSS. That can lock you into HTML structures that you might need to change later on. Honestly, I’d prefer if microformats used a different attribute altogether, but I think that ship has sailed.

I haven’t mentioned anything about microformats in my book, though I could certainly point out this principle in more general terms — try not to use the same classes as hooks for both JS and CSS, for instance (unless the two are inextricably linked to the same component).