when i heal tank i go for sos build and have shield on tank whenever its possible, and most healing done on tank comes from shields, so i wouldnt say mastery is worse than haste or crit for tank healing. and just to clarify i dont do just bubbles.
worked like that in bot/bwd on heroic modes and on shannox in firelands, unfortunately on other firelands bosses i had to heal raid, and dont know about rag fight yet, maybe it differs then or perhaps it depends on playstyle.