Interesting. I been recently playing my 70s to do the IQD dailies: Prot Pally, Arms Warrior, Combat Mace Rogue, and a Holy/Disc Priest.

Paladin Keys:
1: /castsequence reset=30 Seal of Righteousness, Judgement
2: /cast Judgement
3: none
4: /cast Exorcism
5: /cast Holy Shield
H: none

Warrior Keys:
1: /cast Slam
2: /cast Mortal Strike
3: /cast Thunderclap
4: /assist focus
5: /cast Charge
H: Move Forward

Rogue Keys:
1: /cast Exiscerate
2: /cast Sinister Strike
3: /cast Cheapshot
4: /assist focus
5: none
J: Move Forward

Priest Keys:
1: /cast Shoot
2: none
3: /cast Smite
4: /assist focus
5: /cast Prayer of Mending

My key mashing after acquiring a target is: 5, 4, 3, 1, 2, 2, 1.
Adjust the melee via H and J. But the mobs are usually dead by then