My strategy on Leveling Up the characters was to use the ones with the lowest stats. I made a chart and ranked accordingly. And three with lowest rank would become my party. Then when I unlocked a Sphere on the Sphere Grid I would just change the stat on the chart, and if the final rankings changed, then so do my party.

For boss battles, though, I used every member. That way each member would benefit. Right now on my playthrough, I have Tidus, Yuna, and Lulu.