Healing assignments for the win, absolutely. Not that being one of three healers on just one tank brings me to the top of the healing charts, but that doesn't matter. Assigning raiders or groups to be responsible for is great, not only do you know who to keep an extra pair of eyes on, it also fights overhealing. Mana is good during our current raids, but this can change easily if we do ffa healing in the future.
One example is Hexlord in ZA, the bolts have to be healed through with group assignments for sure. Only heal your assigned raiders and trust that others will do the same. It worked perfectly last time I was there.