I think that it's easier to give exp to all the characters.... I just kept switching all of the characters in and out during a battle.... It worked really well for me!!!!

Yeah, i just use Wakka, Tidus and Auron and trained them as much as i could, but its always good to train another character is one of ur mains gets low of hp or confused, posined and such

what i try to do is find the best three characters like mine are auron wakka and tidus then just train them all the time

I tried to level up all my characters once, but as you said it is a hassle. If I really want to talk the time I'd do it. The only person that's not in my regular party (Of Auron, Tidus, and Kimahri) that I try to keep leveled up is Yuna.

She has some handy healing abilities and her Aeons are good when your in a fix.

Well, technically if you just level your main characters you'll be able to beat the game. The game never puts you in a position when you'll need a character critically. Examples of this would be fighting Biran and Yenke with Kimahri. I didn't level Kimahri much and I was still able to defeat them easily. The same goes for Evrae Altana, with a very special trick even if Tidus, Wakka, and Rikku are your most underdeveloped characters, you'll still be able to beat him easily. However, if you want to get the most out of the game and like to complete everything maxing out your characters is a must. I didn't do it on my first playthrough, but right now, I'm playing an earlier save of my game, and I'm dedicating myself to the task. It's proven to be quite fun.

I kept everyone pretty fairly leveled up except Lulu and Rikku before I beat the game. I used a main party of 5 overall and then in the Monster Arena I souped up Tidus, Wakka, and Yuna to ultra-powerful overall so they jetted ahead of everyone else in terms of stats.