I use:

11-22 Shadowfang Keep (can do Scarlet Monastery at 20th, but SFK is still decent speed at 20-22 and there are A LOT of SM runs regardless).
-- Can park your toons at the gate, immediately beyond the quest givers...
-- Only need to have them on follow, once they can do the quests... and then park them at the gate once those are done.
22-36 Scarlet Monastery (Cathedral/Graveyard side till 26th, quests on both sides at that point, then Cathedral/Graveyard till 36th; Armory/Archery even is annoying, I only do that once for the quests which are available at 26th (on both sides))... slaves on follow throughout.
36-44 Scholomance
-- Run the slaves down the steps from zone in, stop immediately at the edge of the stairs (on the floor level, but almost touching the stairs still); they get experience for the whole dungeon from there.
-- Only have them on follow for the quest line; beware the first boss aggros from a huge range with lowbies on follow.
44-48 Stratholme (Dead Side... through the service gate)... slaves on follow throughout.
48-58/60 BRD (Can head to Outland at 58th, but I prefer 60th so I don't have to be slaughtered 5-6x corpse hopping to the Burning Crusade gate more than once)
58/60-62 Ramparts.
-- Can park the slaves at the near end of the first bridge, and they'll get experience for the whole dungeon; only need them on follow for the quests once.
62-64 Blood Furnace
64-65 Auchindoun - Mana Tombs [North] (Slaves on follow throughout; break a short distance from bosses, kill them, re-follow through)
65-66 Auchindoun - Crypts [West] (Slaves on follow again)
66-68 Auchindoun - Sethekk Halls [East] (Slaves on follow; can head to Northrend at 68th)
68-69 Auchindoun - Shadow Labs [South] For the achievement; cannot take the quests until 68th.
69-71 Utgarde Keep... Slaves on follow.
71-73 Nexus... Slaves on follow mostly, re-gather whenever you drop them off; beware the wandering guys who go up and down the connecting halls, if you park them anywhere.
73-74.5 Drak'Tharon Keep... Slaves on follow.
74-76 Gundrak... Slaves on follow.
76-78 Halls of Stone... Slaves on follow.
78-80 Halls of Lightning... Slaves on follow.
You can park the toons at zone in, for: SFK, Scholomance and Ramparts.
You could pull most of the zone to zone in, for Scarlet Monastery.

You could do about half to maybe 2/3rds of Stratholme, with the slaves parked at zone in.

If you go with BRD, it's way to big to not have the lowbies on follow.
Even if you do, you can break follow with a spread out key, kill stuff, use slave only forward movement (into follow range) and start follow again, until closer to enemies where the cycle starts again.