On a PvE group, I drive with the tank.
Doesn't matter whether the group has melee or ranged primarily.

In PvP, in combat I drive with the healer toon.
But in non-combat, I lead with any of the DK's, because they're the same speed but the Pally is faster...
If Crusader Aura is on, I also find it better to not lead with that toon.
For my Ferals, which is 5x the same spec... I generally lead with "Ualaa", but mechanically they're identical.