I've had to stop a few times to level my team up to be able to continue. I've probably levelled up too much at points, as when I go back to the story bosses, they're fairly easy. The real level "grind" would happen when you want to do all the side questing for the Hunts and optional Espers (Summons). You only need to be mid-40's to finish the storyline apparently, cap is 99.