I have had the best luck at lvl 63, myself... It's the level that you get 2 of the DoTs at the same time, and it's right above the lvl cap for the higher lvl instance spawn. At lvl 63, there is only 1 lvl of mob that can spawn below the 5-level spread exp limit. On occasion, a lvl 57 spawns so you only get 1.5% aa or so, but everything else is 58-61 so you get the full 4% or so of aa per kill.
At 65, yeah you get the new fire dot (which adds about 5.5 dps in the long run, big whoop), but now you are killing mobs up to 3 levels below you at a slightly faster speed, and if you really do the math, it doesn't make sense to go past 63 until you buy all the aas you want pre-64, and bank 30 more.
To summarize, stop at 63 and grind aa till you have everything you want that you have access to at that level, bank 30 points, then move on to 67 or whatever post-65 level it is that you get 2 new dots at the same time. Stay there until you have everything you want, then stop at 69, train the rest you want from there, then head on to 70 and train whatever is left if you want to.
My bard will never see 70, because there are no aas that are really that good that require 70 and I really only want him to feed aas to my main
-CodeCaster