I say best time is at 80. It'll get you to 85 and max out your arch. Plus you wont have problems with mobs that you run by.
I found its actually good even if you dont want/need arch on a character to pick it up and use it to level 80-81 then start on hyjal, things go smoother and if you are using rested you can ding 85 in uldum then
I quest my characters to 60 then ArchBot them till 80 and then use Kickazz's Cataclysm quest profiles for 80-85. Works like a charm!