I think it depends where you want to be when...
Learning to tank is not done by solo questing. You need heroic instances/zones with a fairly balanced group setup to experience the strengths and weaknesses of your class and learn how to deal with them. Who to moderate, why Warlock hate transfer doesn't keep them alive etc.
Depending on your server it will be hard to find groups for heroic zones and instances before T8. Once you are there you can start tanking for groups and learn what to do when.
Starting with the original RoK instances it shouldn't really matter whether you have 140 or 170 AA's. And remember you can always return to older content and do grey quests for the same amount of AA exp.
I would solo quest to 80, start lfg for RoK instances (Crypt of Agony and Vault of Eternal Sleep are good starters, Deep Forge and Scion of Ice in TSO as well), do grey quests while being LFG or while you organize your own group (don't enter without decent hate buffs (increase or transfer)), do your fabled epic...
I think you earn more experience (player skill wise) as a Tank while you tank. Having 10-15 AA more doesn't really matter that much when you start tanking for groups in easy zones where you should learn the basics
