Originally Posted by
espoire
Different groups do it differently. In 10man raids, you almost never have 1 healer per tank. Actually, in 10man, Flex and LFR, it's usually "heal whoever you see." In 25s, or highly-organized 10mans, either a leader will tell you who to focus on, or you'll negotiate who to focus on with the other healers.
If you're given free reign, or if you have some say in what you heal: play to your strengths and play to the encounter. I don't play a Resto Druid, so you probably know what those are better than I do. If you're good at rolling HoTs on the entire raid, do that. If you're good at keeping a tank topped up, do that. Always be prepared to heal differently if the encounter demands it though, because some fights will demand unusual healing styles.