I just dinged 4 of my 5 shammy team to 50, 1 is still hanging out at 40. I agree with the above poster. Here's what I found:

If you have a 70, take your 70 and 4 of your multibox team and just let your 70 own the instances over and over. Your guys will get full xp for each kill. You can do the quests as well, but I'm pretty sure that grinding instances is going to get you the most XP/hr. This is what I've found. I did a mix of questing and instancing up until 35ish where then i exclusively ran instances all the way up to 48. My plan is to just go and solo level the 5th man that happens to be on my main's account so I can't power level him just yet.

Also consider that even if you are extremely efficient with your questing, mapping our your path and using all the questing helper addons, you will still be slowed down by people wanting you to explain what you're doing. In instances you hardly ever get noticed and as such things are a lot faster and you have a lot more freedom to stop if you need to, adjust your macros and such, without worrying about someone coming up and harassing you.

I run 5x shaman, but I've read numerous times that pallys make better tanks than warriors for multiboxers given their efficiency tanking multiple mobs at once.