Cure I should be tossed aside once you're in the mid-30s and have Regen.
If you're in a situation before lvl 52 where you have to spam the tank with back-to-back Cure spells alongside keeping Regen up, and you're out of MP because of it, then you probably have a paper tank and Cure I still won't help you in that situation. Teki is right, Cure I should be avoided once you have Cure II and any healer worth their salt will tell you this. GCD efficiency is much more important than MP efficiency in this game.
Also Holy stun does far more for a tanks health than any heal, it's practically a 7s invuln every pull. I always tell new WHM players this if I notice they're at a level where Holy is available and they're still relying on Cure and Freecure.