Aggro needs to be monitored throughout the fight when in dps stance, there really isn't a measure for how much is enough, as once you switch stances your natural enmity generation will decrease while healer and everyone else keeps going up. So it's best to just keep an eye on and add enmity combos if it starts getting to close. And make sure the tanks are shirking when appropriate, healer and dps using threat drops, etc.

I don't have any experience raiding on drk, but can tell you mostly without a doubt it will be easier on war simply because they have more CDs to rotate.

As for war not needing an enmity combo. You really need a ninja to help pull that off. I could probably go without butchers but I do it in my opener anyways because IMO I'm going to have to do one or a few sooner or later, so I just do one in the opener and more if I need to later on (tho I sometimes do more than I should, mostly due to my controller setup)