I can go almost either way... I often find it more natural to include a (s)css file in my component's .js, and have webpack bring it in... then in the component's rendering set the className to a matching container in the scss. With scss, I can start with my variables/mixins include, and keep the more hierarchal stuff straight... It works pretty well, but I see the appeal of things like this library, aphrodite and the like.

I do exactly that too :-)