I highly recommend that you do instance powerleveling from a L70 until about L38 (or 40 if you get some class-defining skill there). After level 40, it becomes tedious, expensive (worse with melee classes, obviously), and marginal in terms of efficiency compared to questing.

At L40, I switched over to a quest leveling guide (plenty available on the internet) and found it much more fun and on par with either mage or prot-pally runs through instances. I still have a L70 with me at times, but it's mainly to knock out some quests faster/safer (I'm on a PvP server) than if I was with the lowbies. 40-Outlands goes really fast with the post 2.3 changes, plus you get a chance to start refining your macros and strategies.

I definitely recommend getting to 40 in instances with a high-level character (can't beat the SM series...), but after that, strongly consider going it "alone".