Appropriate times to use Clemency?

[Saeno]

1) Progression. If you are still learning the encounter and how to best tackle it, you should be using Clemency often because your DPS doesn't matter when the boss isn't going to die.
2) If you're going to die, use it. Dying is a much greater DPS loss. This goes for anyone else. Saving them is more DPS and it promotes good PLD play as saving people is something your job allows you to do.
3) It's a very powerful skill that is very underrated. It's ok to use it if you feel like it would help. Don't be afraid to use the full kit of your job despite it not contributing to DPS immediately. Use your own judgement.