Ultimately, its most efficient if you do a given quest once, and then you're done with it.
A lot of the collection quests, which are a pain to do for five toons, open up subsequent killing quests.
If you collect with only one toon, then the others cannot do the follow up.

In the end, it doesn't matter how you get to 80th, as long as you enjoy the process; it is a game, afterall.