I like to make sure I cover Grass, Water, and Fire. Usually a Flying-type as well, though that's more because Fly-learners are almost all Flying-type anyway. I also tend to end up with a Psychic-type. Other than that, it varies, really. I worry more about what I wanna try out, what I just end up getting, and what I need for HMs.
"Why don't you just take this picture for what it is? Teemo being stuffed and mounted by renek." - Serenderpity LoL Name: Thanamar
A Grass type, a Fighting type, something that can fly and/or surf (not necessarily matching those moves types either) and then after that it is whatever I can find that I like. If I can, I'll stuff all of the convenience HMs or the ones that are fairly usable in battle onto one or two Pokemon.