If you have 4 or less, Instance powerleveling with a L70 AoE-capable class is by far the way to go until they hit L40.

5 becomes complicated because you can't easily use a L70 to help. You CAN rotate through your other characters if you have 6 accounts, which won't slow things down terribly much. If your 70s are also on the 5 accounts, you are forced to maybe level 1 by themselves and powerlevel the others.

In any event, I don't think instance powerleveling is worthwhile post-40. ZF becomes too tricky, the exp/hour isn't vastly superior to questing anymore, and it just isn't fun, to be honest. Overland questing and/or running level-appropriate instances (if you have 5 that can do it) will serve you much better. Post 2.3 quest XP and dungeon quest XP is very nice and will help you go from 40-58 very fast.

I advocate Outlands at 58 to get the great quest gear, and I've never had too much problem with quest gaps this way. You may have to run some instances or grind a bit in your low 60s, but it sounds like you're trying to put together an instance-capable 5-box team anyhow.