It would really depend on loot imo. If you have a tank that doesn't need anything off a particular boss and your raid would benefit from having some extra DPS for the fight I'd say go for it. For the most part I feel like it will be situational.
We typically have four tanks in raid at one time: 3 that can go off-spec DPS and one that goes holy. If we happen to have people on sit out for a boss our RL will typically ask the raid as a whole to send him a tell if anyone would be willing to sit out for someone else to jump in for a kill or two.