The biggest problem I've had dungeon boosting is when you can just wiped through a dungeon so fast you hit the 5 instances per hour cap. I run alliance toons and help friends level alliance toons so I generally don't take them to ragefire chasm. Typically by the time I could run my lock and two others in to a do a summon of the low level toon, they could have quested from 8 to 10 anyways. So I do:

10-15 Deadmines <- I occasionally hit the 5 run rule here
15-20 Stockade <- I always hit the 5 run rule here
20-35 Scarlet Monastery Cathedral <- I have hit the 5 run rule here before, but it's rare
35-45 Zul'Furrak
45-55 Dire Maul:East
55-60 Blood Furnace

Then I go back to levelling. I'm curious though what the minimum level to enter some of the WotLK dungeons are and what using those to boost would be like though. Nexus maybe? Don't know haven't tried.