Yeah, you shouldn't have to get any in the last dungeon though, so long as you got them at the first opportunity. The only thing harder than the last boss fights is Omega Weapon. Personally I'd get them all just for the magic refining abilities (you can have so many Ultimas!), but you can still be ridiculously strong even if you're missing a couple. Eden's Devour ability can actually raise your stats if you use it on the right enemies by the way!