I'm going to do whichever is most fun... I will definitely start out leveling my 5 shamans and if I get a bit sick of leveling them I might bring my 64 mage in and use 4 shamans to power her in outland some, then switch back to my shamans at some point. I just play whatever i sthe most fun at the time whether thats 5 characters or 1, no one can tell you what will be the most fun for you.

I feel that everyone will have more fun with this game (and make it feel less like a second job) if they just do what is fun and stop trying to min/max every single thing they do.