This started as a response to an Ego post on downranking, but grew well out of scope. The short version:
For raiding, you do have to offset damage taken against the healing you expect them to receive from other healers.
For raiding, the key points are:
#1. There is a minimum amount of overheal required to keep a raid tank alive
#2. The goal is to run out of mana exactly at the end of the fight
#3. Mostly, healers have SICK amounts of regen now. We aren't in MC any more. Eldr has nearly 600 mp5 oo5sr and he's far from leet.
Healing by prio is very effective:
#1. Can you CoH 3+ people?
#2. Does anyone need a shield+pom?
#3. Does anyone need a flash?
#4. Is PoM on CD? If not send to tank.
#5. Is a tank healer having trouble keeping up? GHmax
#6. Is renew up on tanks?
#7. Do you have <10% mana? Take a spirit break
#8. Chaincast GH1, even if the tank is full
Besides this, there has to be awareness of the other healers. Paladins are spike saviours, let them do it. Druids prevent spikes, Shaman spam CH4 - help with spikes more. Even if you have paladins, if there's spiky damage on ranged they may be FoLing, watch where others are healing and be cautious - better to waste mana than lose the tank.
We rarely allocate dedicated healers (and my priest is an alt), so I prefer to help out all over rather than try to muscle in on a specific area. Everyone is better at something than priests, I see it as my job to keep the toys going (coh, pom, renew) and help out as necessary. When flashing, I often target the person second-most in need of a heal, as my heals are smaller than others' and the other healers are very, very fast!