5 all the way I've run instances with friends, and even though one of my toons may fall behind a little, I just spend the next time I go questing, doing some collection quests with just that character and he eventually catches up (Ie Kill quests on all + collection quests on the one that didn't go to an instance

Plus it means you can go with friends or on your own

as far as when you reach lvl 70.. well if you have all the same class then just alternate the toon you leave out, that way you will end up with roughly the same amount of tokens