I use them all equaly so if I am forced to use my 'reserve team', they won't get utterly raped before the other guys are revived and ready for action again.
I not long ago started a new file. Right now I'm catching up on some side quests, hunts n'shizzle before heading across the Sandsea to Raithwalls Tomb. Possibly the single most boring part of the game tbh
Heres their main weapons and role in party:
Vaan = Swords and shields (Tank, Technicks)
Penelo = Daggers, Bows and Staves (Black Mage)
Balthier = Guns, Daggers and Shields (Thief, Green Mage)
Fran = Bows, Spears (Damage Dealer, Time Mage)
Basch = Greatswords (Damage Dealer, Arcane Mage)
Ashe = Bows and Rods (White Mage)