DnD is great for AoE threat. DnD and HB are both amazing AoE tanking tools but they shine differently. HB is great for snap aggro but if it misses a mob they can run right through to a healer or DPS. DnD doesnt have that snap threat gen that HB does but it can pick up passerby's and over the length of the spell produces a large amount of threat.
I take it that you're a Frost DK. Being frost you can easily AoE tank only using HB while juggling diseases with Pestilence and I would suggest doing that. When I'm frost I initially pull with HB, if I have death chill in my spec then I make sure it's a crit on the pull. I then follow up with IT-PS-Pest and then HB again when it's off CD.
If you have the abilities and tools use them to their fullest, your judgement seems to be spot on so far.