I just pluck when I feel like they need it. The shape is good, so it's really just a minor clean-up, and I can get away with not doing it for quite a while. Otherwise, if I'm trying to look more 'polished', I'll fill them in a little bit with some powder/matte eyeshadow close to the same color, and an angle brush. My hair is like a dark golden blond naturally, so my eyebrows aren't very dark. A little definition helps them stand out more.
Sometimes, I pluck them just a little bit since they're already shaped and I used to fill them in with brown eyeliner. I don't use eyeliner anymore because even if I fill them, it doesn't look right for me. I just put olive oil on them every other night.