Don't bother with 5 shammies, perhaps. Just do 4. You can use an existing tank to rapidly powerlevel them, and you'll find that Tank+4xShammies is a great PvE setup. 4xShammies plus a healer buddy will also serve you well in Arena.

For BGs, I vastly prefer 4xShammies over 5xShammies for all maps except Alterac valley. 4xShammies is enough firepower, and leaves another crucial slot to be filled by another player for maps like AB, WG, and EotS...