They only way I have ever found grinding to be feasable is when you have a higher toon to kill quickly and normally you are boosting in instances to offset the xp reduction from the boosting toon.

as for normal world grinding (non instance) I would not even consider doing this past 60 and onto 80 as this would take forever (bearing in mind that 90% of the kills i have made in Northrend starting zones only give between 430-550k per kill), but the main turning point would be that without questing none of the dailies would open up at later levels