There's nothing wrong with using it after heavy AoE to help out the healers. Unless you're doing content where you actually need optimized tank DPS (which, you know, you're probably not, given that you're on the forums asking when to use one of your skills. Just saying), there's no reason not to try and find ways to utilize your kit.
Your job as the tank is to make the run as fast, smooth and comfortable as possible.

...oh, and if the healer complains at you for using Clemency? Screw 'em. As a PLD in level 70 dungeons, having a healer is a convenience, not a necessity.