In my opinion, it depends on the type of player you are. If you are a real powergamer, and intent on getting the most amount of experience in the shortest period of time, the most -efficient- route is going to 65 straight with no AA, then turning on AA to full and start grinding. I say that because if you grind at a rate where you level quickly, AA will not significantly increase the rate at which you level compared to the time it takes to get it.
But...
For most players, getting from 50ish to 65 is going to be fairly time consuming, and sometimes boring. Most players won't do 50 to 65 in 3 or 4 real life days, and not notice the difference. I'd get a few AA to make the levels feel less annoying, and see some constant progression for the fun of it, unless you are extremely goal driven and are just going to push for 65 around the clock.