In LG, I never made a true team. I just caught and trained every Pokémon, and rotated them in and out so that almost every Pokémon I owned was at the same level.
In most games where I do make a team, I don't finalize my team until after I beat the Elite Four. I find it easier to stick with 3-5 Pokémon so that I have enough room for HM slaves, Pokémon I want to train to complete my Pokedex, etc. When I do make a team to use in-game, though, I generally do it with Pokémon caught by the second badge or so, and then often add a late-game Pokémon when I get to that point.