I stopped levelling my toons together from lvls 1-20. The xp per kill wasn't that great and it was a serious 'grind' for some of the item collection quests.

After 20, grouping your toons together seems to be a better idea. Especially with the new (after 2.3) improvements to levelling.

After 30, no contest... multi-boxing all the way.

I think it's posted somewhere here, but there used to be a chart showing XP rates. The chart showed from levels 1-70 how much xp you got per kill with and without rested, in a group with 1/2/3/4 others, etc.,.. I don't recall the chart displaying xp grouped with diff levels or whatever though.
(I can't seem to find it, but I'll continue looking.)