Like in most of the final fantasy games i play, i try to make all my characters equal so that, if one dies, i can always switch in another character that is as good as the one that died. But it seems that near the ending i have to end up picking which characters to use since if i don't it would mean even more endless training to make them good enough for the next area. Now in FFX i didn't have this problem as bad as i did in other ff games such as 7. But by the end i stuck with Auron, Tidus and Yuna.