Either way, disc priests will be one of the last options for a raid to assign a healer to heal the tank. At most, a disc priest should ever PW:S the tank whenever rapture is needed or if the tank is in danger of cloak proccing.
Resto Shamans and Holy Paladins are way better at doing that because their main tank heals are exactly the same as or don't detract from their raid heal rotation.
As a healing officer, if I see that the tank requires extra attention, I tell my Hpally to just select the tank as the main target for Holy Radiance, and if he's not in the raid, I tell our Resto Shaman to Chain Heal off the tank while maintaining Riptide. Nobody else in the raid needs to specifically heal the tank as long as this is called.
Think the only time where I ever used defensive penance was for a blood dk whose purgatory procced.