Going into MoP I think you'll want to run the characters as whatever spec you want them in finally, even while leveling, due to the way quest rewards are being handled now.

For example, my first team will likely be 5 x paladin but while leveling I'll be going with 1 prot, 1 holy and 3 ret just so when I hand in quests they get the spec appropriate gear. If I ran all 5 as ret then they'd only have the choice of ret rewards and if I decided to go level up in dungeons for a while I'd have to go and buy holy and prot gear for the tank and healer.