I also want to point out that it is perfectly fine to nuke in dungeons, but you will want to practice it and take a few battles early in a dungeon to get a feel for how long you can let the tank go without healing. Not only will this make the dungeon go more smoothly, your Limit Break bar actually fills up much faster if you heal people that are closer to death rather than spamming healing when they are near-full. Once you get the hang of it you can throw in Cleric Stance switching and then healing becomes a very active role.

I only say this because endgame dungeons will go a lot more smoothly if the healer assists in damage when the party's HP is fine. The dungeons I have done required you to kill enemies quickly to get away from a pursuer, and a few bosses have required me to throw out a Holy or two in order to get rid of assisting enemies quickly before they can become an issue.

Some fights are definitely intended for you to focus on healing 24/7, though, but it will be obvious when that is the case.