I'm guessing my starter (Samurott) on my main file will be my first level 100, as that's what I normally do. I still haven't actually starter B/W yet (I should get to this weekend, finally--it feels like it's been a lot more than two weeks or so since they came out), but I'll probably try to get one with a good nature and decent IVs. I might try to EV train it a little, but not too much (since it's hard to EV train starters at the beginning of the game).
After that, most of my level 100s will probably be custom-bred, EV-trained Pokemon with good natures and IVs.