There is no problem with phasing, I just don't think they will phase a faction leader.
---------- Post added 2013-04-03 at 11:12 PM ----------
Only for a high level character. And like I said that's not the normal leveling flow. No one is send baclk at level 90 to do a level 90 quest in Northrend. A level 70 character questing through Northrend is perfectly normal.