In packs of 3 or more I'll cast Repose on the first pack and see what happens. If I'm with a WAR usually it's not worth doing as they're going to use Overpower. With a PLD, it'll depend on how aware they are I'm doing it and whether or not the other DPS does single target damage or not. After a couple of pulls I'll have an idea of whether or not it's going to be worth my time to do it or just prepare to spam heals on the tank.