I mix it up. My most common MO, though, is to find a stereotype/archetype and turn one peace on its ear. I guess it lets me feel like I'm using an archetype and not a stereotype.
Best example: I wanted to play a Hermetic in a M:tA game. I had noticed that pretty much all the other players tended to play middle-class white characters -- just like all the players were in real life. Running with that, I turned the Hermetic into a black, ghetto-born drug dealer. The fun part was figuring out why he was also an intellectual.
Sometimes, though, I will build a character around a single concept, like the pacifist I played a few years back. Or the transmuter who had a hard time differenciating sentient creatures from inanimate objects.
Other times, a personality just forms in my mind and I build the rest of the character around it without really factoring in stereotypes in any way. Those are the rare gems, though.