Do you ever wait to level a character until after...?

Omnirhyze

Promoting it a few times? I notice the growth modifiers increase as their stars grow, so when you level it up, their strength, intelligence, and agility will increase more than it would've at the previous star.

FLIK87

All the changes are applied retroactively so you can Gear, level, star and add ability mats anytime you like!