Well, once per day, might be worthwhile, as you're in the leveling up stage.

Northrend at 68, will be faster then Outland.
But if you want maximum gold, then stay in Outland until quests grey out.
You'll have a lot of quests to do, at 80th.

The flying is very nice, but the game was doable prior to having cold flying books to mail.
In the end, the toons are yours, so you make the call.