i'm using everyone, so i divide the party into 3:

main mages (w/mystic armor):
Vaan - White mage but knows most black magic as well, with Spears
Penelo - Black mage but knows most white magic too, with Daggers

secondary mages (w/light armor):
Balthier - Time mage, equipped with Bows at the moment
Fran - Green mage, equipped with Axes but uses Bows when needed

warriors (w/heavy armor):
Basch - with Swords, Technick master
Ashe - also a Dark mage, equipped with Poles or Crossbows

so essentially, other than boss battles where i might switch things up, there's gonna be one rep from each group at all times