Depends on what your roll is in the raid. If you are MTing defiantly don’t use it for Frozen blows but feel free to pop it for Tantrum or in phase2 of Mim. Of you are OTing I use it as another externally cooldown for the tank, usually after HoSac.
It really is a good spell. Basically a raid wall, it can massively reduce the amount of incoming damage that needs to be healed. And yes always blow Divine Shield (not protection) before it.