So here we are, 3 characters in northrend and a 4th slowly crawling to that magical number of level 68 and now the time has come to determine whether or not it's worthwhile to bring number 4 - 10 to northrend at 68 or to put off till later (level 70+).
I am not going to say one way is better than the other, that varies per person (yes it does). What I can do however is list the facts and let you come to your own conclusions.
And thus, I present to you the pros and cons of going to northrend at level 68:
1. Higher XP/gold gains for quests and kills
2. Higher valued quest rewards
3. Higher grouping potential
1. Higher risk
2. Reduced max gold potential from questing at 80
3. More competition
4. Outland loremaster quests won't give XP
That's essentially it. What it comes down to is a choice between northrend's higher xp rewards, more valuable quest rewards and more grouping options vs outland's low risk environment, lower competition for mobs and the ability to squeeze the maximum amount of gold out of northrend quests later on (the more you quest in outland the more quests you can turn to gold at 80 in northrend).
Is one better than the other? That is a matter of personal preference and I am sure you will find people on both sides of the fence.
As I have pointed out in a previous post however is that ideally you'd want to ding 80 after completing Borean tundra, howling fjord, dragonblight and grizzly hills in order to get the maximum amount of gold out of northrend at level 80 for the remaining quests.
If you're interested in that and still want to go to northrend at level 68 consider taking a little break from questing and focussing on dailies for a while.
They give decent XP, decent gold and are finished quite quickly but can provide enough of a buffer to get you to ding 80 earlier than you normally would by just straight up questing through northrend.
Incidentally if you have any intention of becoming a loremaster then you'd be well advised to finish outland while there's still XP to be had from the quests.
The choice is yours... and while it's probably not interesting for your main to ponder there's always alts. Yes, that off-gray pile of dust on your character screen may in fact be a long lost tauren just waiting for you to take him/her to northrend... You're not just going to leave it there are you?