The owl selector (*+*) is a great solution for bottom margins. It selects every element with a matching preceding element.
All the studies I’ve read, including Steve Souders’ and Ben Frain’s, have concluded that the comparative performance of different CSS selector types is negligible. In fact, Frain concludes that “sweating over the selectors used in modern browsers is futile.” I’ve yet to read any compelling evidence to counter these findings.
It’s all about file size.