In the beginning i only used Auron, Wakka, and Lulu, but then I realized that it can pay off big time to use Tidus and Yuna alot because Tidus can learn Haste and Slow, and if Yuna isn't used often the aeons don't get stronger, which are an extremely valuable weapon against bosses. By the way, Rikku is pretty much useless, except for her ability to use Al Bhed potions, which cure all status ailments and restores 2,000 HP of all characters, that's about all she's good for.