This is an interesting topic. I'm close to L20 with my 5 shammies and since I'm thinking about rerolling on Mag I might do a blended group as well. I think its ultimately going to make life easier at level 70 with instances, though obviously its a much tougher path for multiboxing itself, due to the sheer number of abilities and synergies that have to be considered. So here are my "guiding principles":

  1. Start with a Paladin as a tank. This is simple.
  2. 2 Elemental Shaman for DPS and multiple totems.
  3. At least 1 mage for water and porting.
  4. Either a druid or priest for a different flavor of buffs. +Stam versus GOTW basically.
So ultimately the question is the 5th slot. Mage for a 2nd sheep? Druid for dots and GOTW? Priest for healing and +stam?

Fortunately I'm still building my new machine so I have time to think about this.