I think its best to address this question at character creation. Have the players over for a pre-game session that's only for PC creation. Talk about concepts, let the DM give his/her view of the game, ask questions, etc. The result of all the jabber should be a group made with at least a nod toward consistency and cohesiveness (maybe everyone is kin, or all the same race/clan, or old friends).
After that, though, I think its best to let organic character growth happen, regardless of whether or not IPC happens.