Yep.
I have Vaan, Balthier, and Fran at lvl 24 right now (just got out of the tomb) and the rest are at lvl 18. I don't really care to level the others but I do take them out sometimes.
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I level them all equally, it's no trouble to me. I have all six of my characters at level 44 right now, and the license board is basically complete for everybody. By complete I mean everybody has all the Augments, Magicks, Technicks, and Quickenings. I have everyone on a different style weapon for the most part. The best part of leveling everyone is that you can pull off a chain of level-three Quickenings with your three party members, and then rinse/repeat with the other three.
It would feel weird to level only three party members now. I like to have an uber all around team.
I use everyone. I just feel like keeping everyone equal. I can not explain this.
I only leveled up three characters (Basch, Penelo, and Ashe) because I realized that even the characters that aren't in your active party get LP (and because I'm lazy).
So once I get done with the story I'm gonna have blank slates with thousands and thousands of LP to experiment. Yay replay value!
I generally use Penelo, Fran, and Balthier. But Ashe and Vaan aren't a huge distance behind, if they need to be in a fight I can bring them in. Basch... eeeh... let's just say he's not as awesome as he was when I first got him :p
I leveled up everyone equally up until thirty. Then I realized that it's pointless--I'm taking twice as long to level my characters. Now I use Vaan with Ashe and Penelo because the girls have the highest innate magic and Vaan is well-rounded.
Vaan, Basch, and Ashe. With spears. Total beatdown.
Ashe have low HP and Penelo sucked in Strength and Vitality.
From the beginning till the end of the game, no one will leaves your party except in towns. They never miss anything. I have my Basch, Ashe and Vaan, all level 66, with Axes/Hammers, all magicks/technicks learned, act with all of the job in my first and second data.
But I use every character equally in my third data after I know the terror of Rank VIII Mob Hunt and I don't care, I have the patience to bear it. I have way too many problem when I only use three character but I'm fine so far using all character.