I use 4 stylesheets:
- reset (duh) - layout (structural stuff) - typography (general typegraphic styles) - styles (stuff gets prettier than the already are).
As for the grouping o selectors, i try not to reuse much code so cascading will not mess stuff up, i'd rather make big css than fragile css. I also try to group code on a per section basis… just remember i do have a typography stylesheet which makes most of the stuff readable and pretty before the glitz rolls in.