Paean is basically request-only. There are some guidelines to guessing when to use it if you want to be pro-active, but they aren't very reliable.

For Foe, you can basically use it any time that a BLM/SMN/WHM/SCH will get use out of it. The only thing to watch out for is if the healers wind up needing MP. That said, I'll usually either use it early so that my MP is back before the healers need any, or, if I don't trust the group to not strain the healers, I'll just hold off on it entirely.