If you are 3 healing on progression, then CoW is probably not the way to go for soaking doomfire.
If you are 4 healing, then anything goes, with CoW being the "safer" option as you don't have the added pressure to heal the raid as much
Words of Mending is really helpful as a pick-and-forget talent to heal through the multitudes of raid damage on the encounter, and quite frankly, when I did this fight 3 healing, there really wasn't any good time to spam CoW.