My DK's (running five also) were primarily Unholy.

Unless you go with Blood primary, you won't be able to reach Rune Tap.
And glyphed Rune Tap was a very nice heal.

I'm toying around with the idea of one Blood spec, to hold threat.
And then one Frost spec, for the melee buffs.
And three to stay Unholy, for the pets.

Not sure that Blood will be enough healing, without a healer.
Was messing around with Taunt, as a round-robin ability to move aggro about.
So only one is taking most of the damage at one point.

The other option... drop a DK from the team.
And add a toon that can heal.
And then you have a DK that can be added to another team.