I'd add one thing: avoid multiple classes on a single element; use mixins to avoid duplication. I tend to do so even for classes that have some toggle state, i.e. .some-class-active.
If you're talking about the difference of:
.a { ... } .b { ... } .ab { ... }
<div class="a b"></div> <div class="ab"></div>
It's probably not going to matter a lot. It's two map lookups vs one, and some associated overhead. Probably better to optimize elsewhere. :)