The tank is always the main windows except for one group : My healing druid + 4 hunters group is controlled through the druid. Mouseover heals are really nice, I wish I could use them in every group ...

For the rest, since you already have the complexity of a 5-classes group, you might indeed try to have as much variety as possible in what characters wear. Farming 4 of something (like mail for the Shamans in my PVE group) is not really fun.

I could imagine a plate tank, a priest healer, and elemental shamans + hunter + balance druid for DPS. Then you have as little overlap as possible in terms of gear.