So far in the game, I have used Lucca, Marle, and whichever third character is required. When the third character is open, I've used Crono. I won't be playing Robo, Magus, or Glenn at all in the game except when their presence in the party is required. I've told ZeaLitY that I'll make Ayla my lead character once I get to that point in the game, and I'll probably continue to use Lucca and Marle, because I love the Antipode spells.
Does the party order matter? I haven't been keeping track of who is in which position, but I can start doing so if it would be significant.