Can You Heal TOO Much?

I got a call from Pottymouth (warlock) asking to come and run a guild heroic. Apparently two groups were heading out to take on Heroic Nexxus and I was needed to heal. I love being in a guild that actually NEEDS me. That is a post for another day. We went in with me, potty, pagan (druid dps), frosta (hunter) and branard tanking. We did pretty well. Branard is flat out getting scary as a tank. I know that I've mentioned it before but the guy is good. We wiped 4 times before we were done mostly getting hung up on this one boss that is a big elemental. He sent these rifts out that we weren't dealing with right away and I was getting pounded on and of couse died. We put him on hold and moved along to the last boss who is a female blue dragon. Apparently Malygos has a thing for female blue dragons. Anyway, we handled her without much of a fuss losing Potty and Frosta in the fight. We went back to the elemental boss and it took just under 5 minutes to take him out but take him out we did. I received the Heroic: Nexxus achievement as well as a pants upgrade and a 'frozen orb' and a nice JC recipe. All of the dps players wanted to know what recount said there dps was. I was kind of curious on what it said the healing was. Come to find out I'm guilty of 'overhealing'. I've always had this opinion that if the mob dies and we don't then we're good to go. As I started thinking back to some of the longer fights my mana was getting a little low where I either had to throw out my shadowfiend or drink a quick mana potion to get my mana pool up.
Part of it I believe is still my healing learning curve and part of it can be that the gear is still in need of improveing. One wipe I think was caused because I was actually trying not to overheal instead of focusing on keeping Branard standing. I found myself doing some research on the 'overhealing' topic in other priest blogs. I need to become more efficient with the mana that I have and more selective with my heals. At this point I put a 'prayer of mending' on the tank and when he first hits the mob I put a 'renew' on him. It doesn't take very long before the POM is bouncing around and the renew is keeping him topped off. With mobs that hit hard for more damage I'm usually throwing a 'flash heal' and throwing another POM and trying to anticipate the size of the next damage hit. Will it be a flash heal or a greater heal? I usually hit the tank with a renew and a pom when the cooldowns fade. When the group is taking damage I usually hit the 'circle of healing' button along with a renew here and there. In desperate situations I have my own direct heal and a 'praying of healing' for the group and 'spirit' for the tank but I've learned that POH is a mana hog. It's expensive mana-wise. There do seem to be quite a few fights were there is a lot of aoe damage so other than COH and POH I'm not sure what to do with the group. I know the heals, timing and mana regen will come with experience and experimentation but I do seem to be allowing to many deaths.