It's just not possible to have on reforge be "optimal" for every fight; the choice is either to reforge frequently between fights, or to find a compromise. Personally, I'm going with Mastery>=Haste>Crit. That way, not only do I take advantage of the many fights with chances for extra embers, I also have a decent reforge for when I go Demonology (on Council).
However, as always you should get your priorities sorted based on what is important for your guild. If you're doing progression on Horridon, reforge full Mastery. If you're doing progression on Jin'rokh, go for haste. Every guild is being held back by something else, so adjust your reforges according to that. During farm, doing a few k DPS more or less doesn't matter much aside from vanity, but for a progression encounter it can make or break the fight.