I would use everybody. And i got everybodys sphere grid max out in 120 hours.

yeah i kind of did that but i hardley used wakka or tidus lol.

I use everyone for the first half on the game, as everyone has their own purpose, except for Kimahri (who I make learn the steal skill, then just follow Auron's grid). As everyone covers more of the grid, I can cut back on characters untill I have three that can cover everything (usually Yuna, Auron & Tidus)

I pretty much balanced my people out towards the end, then when I started doing the side quests, I focused mostly on Tidus, Yuna and Auron, in which they now have 50000+ HP, 2000+ MP, drastically high stats, and fully powered celestial weapons. =D