I jumped on this earlier i actually have three groups i do most of my stuff with and sometimes might change them out for another class.

My world farming group believe it or not is 10 blood dks. Why self heal can pull tons and can stay alive. If i see imma need something different ill add in my disc priests if i want to help dps. Otherwise i bring in resto druids three and just let them heal only.

World pvp/bg will always be my 10 rogues. Its insane in one gcd you can normally drop 2-3 other people. Plus if your going after a group with multi healers, you burn one vanish burn the other premed and vanish. Wod will be nice with backstab no facing req's.

Then its my 5 man team for instances. This group i rarely switch out ever. I use a pally tank and 4 disc priests. I can pull alot of mobs at once pop a bubble 4 renews and just let my dps do the healing. I know after wod there will be a nerf to attonement buuuuuutttt who cares. Right now its 90% damage done goes to healing, i am always way overhealing. After wod it goes to 50% which i might see a tad in the midst of target healing now and then but i doubt it.

Like i said sometimes i switch a tank for instances but never my priests. Pvp its always rogues its more fun. Three man scenerios is whatever i feel like. Havent got a good grasp on the setup for that.