I asked the same question of this community six weeks ago and hit Northrend as freshly minted 68's (four of me that is, my "main" was halfway through 70).

I've done almost enough quests in BT to get the achievement and have about half that many quests done in Howling Fjord. My team is 72.5 now (main is 74.5). I have yet to do a quest in Northrend other than those two starting zones. I've done a little bit of the two initial instances, but just recently.

I think I skipped one or two collection quests only because they didn't lead to anything. So far I have been AMAZED at how UNPAINFUL the collection quests have been as a five boxer. Mobs spawn fast, generally drop whatever the quest item is and if its a ground item collection, the items seem to be plentiful. This may change as I move beyond the starting zones, but so far its been much better than waiting and doing Outlands. To be honest, the ratio of hard collection quest seem to be on par or even a bit EASIER than what I recall of level 68 - 70 zones in Outlands.

YMMV