Sydera has an excellent post over at World of Matticus about healing meters and how to read them.
I very much agree with most of what's said there, except for her views on heal sniping. The comments also bring up the topic of overhealing which I'll address below.
Heal sniping is healing a target that somebody else will heal shortly. This way you get all the credit on the healing meters while the other person only gets overheal and has accomplished nothing.
Sydera claims correctly that a certain amount of heal sniping is unavoidable, especially when raid healing. Clear-cut assignments can reduce the amount of sniping that's going on if they are followed.
Obviously sniping has its disadvantages. By making another healer's heal irrelevant you reduce the amount of effective healing that's been done in a raid which may, in a hard encounter, lead to healing missing in another spot. Sydera highlights the example of the tank healer sniping on other targets and losing the tank while doing so.
Sniping isn't all bad though, on the contrary I would claim that it is essential. A healer that is used to sniping will be much more able to react to unforeseen changes in the healing situation than someone that has strict assignments to follow. As a shaman in the current environment there are really only two roles you can have - tank healer or spot healer. A spot healer is de facto an assigned sniper that drops heals wherever somebody loses health while a tank healer usually spams heals on one assigned target.
If the shaman is always assigned to tank healing the only thing she will learn is how to spam Lesser Healing Wave on one target and keep Riptide on cooldown and Earthshield up. If that shaman has to switch to spot healing for any reason (say there are a holy paladin and a discipline priest in the raid) she will perform poorly simply because she doesn't have the reflexes trained that are needed for it.
Now I'm not saying that a MT healer should be sniping around wildly since that can indeed kill the tank, but every healer should be willing to use lulls in their own assignments to snipe as much as possible - they just need to make damn sure that their original assignment isn't lost.
A prime example would be Steelbreaker (Iron Council hard mode) where I almost always have to heal the Steelbreaker tank - along with two other healers. Now Steelbreaker dishes out some tough damage but his normal melee isn't something that two healers can't heal (at the start of the fight). It can even be healed reactively. His ability Fusion Punch, however, hits for so much that three healers are necessary and precasts almost essential. I therefore keep my Earthshield and Riptide on the tank while helping the other healers with the AoE damage that's flying around (sniping). If I see my tank drop I throw a quick Lesser Healing Wave and if I see Fusion Punch being cast I start spamming the tank like mad.
In short - snipe responsibly and you will not only be higher on the healing meters but actually be a better healer. Your assignment should always have top priority however.
The comments to the aforementioned post talk a lot about overhealing and how it doesn't matter in current content. That statement is false.
What's true is that you can have as much overhealing as you want if you get the job done. Especially paladins on maintank healing tend to have a very high overhealing percentage and that's fine.
In fact - and this is why overhealing matters - having high overhealing when tankhealing is essential! When I am evaluating a tank healer and see that they have low overhealing I know that they are not doing their job. Especially paladins and shamans on tank healing are supposed to spam the tank until the cows come home (or the boss dies, whichever comes first.)
Yes I did say that you should snipe responsibly but that's exactly that - responsibly. If it's your task to spam the main tank, you spam the main tank! If you're not doing that and the tank dies it's your fault. And if I see that one of the tank healers has high overhealing while the other doesn't, I know whose fault it is.
In short, overhealing matters in three scenarios: When you are going out of mana, when people are dying while you are overhealing other people and when you are main tank healing.