When I 5box my DK's through Heroics, I HAVE to keep them all in Frost.
I tried specing 3 of them DPS, and two keep tankage.

I put DPS gear on them, put them in Blood/Unholy, and I could hardly get past the trash.
Because I have no healer persay, any damage they take, or aggro, or AOE damage, it drops them quick.

Put them back in no Crit range/Frost Aura, and back to normal clearing.