5 shamans can only do a few heroics... and can only do it by kiting.

To do heroics effectively, and traditionally, you need a "real" tank. A lot of people like to go with Pally tanks as their AOE tanking ability is far superior to Warrior and Druid.

Pally Tank + 3 Mages (CC) + Holy Priest
Pally Tank + 2 Mages + 1 Warlock + Holy Priest

Those are 2 of the more common Heroic-centric groups discussed. They provide plenty of DPS and CC. If you plan on PVPing at all, keep it more simple with the Pally Tank + 3 Mages (CC) + Holy Priest combo so that you can take your 4 casters into a BG.