I think someone else stated this, but for 3+ mobs I will drop DnD, HB, BB, blood tap, then bb again. IF deathchill is up, i will pop it before HB for some nice numbers. If the CD that refreshes runes is up(cant remember the name lol), i'll use that and then do the usual dot and spread stuff.
Nowadays if u need that xtra umph of aoe threat as a frost tank, you should def throw down DnD. IDK bout u but the dks in my guild love DnD, and with wandering plague they draw some nasty aoe threat. So i need to squeeze any little bit of aoe threat I can.
I agree it sounds like dps aren't waiting for you to get the mobs in position and use HB, with frost fever up howling blast is great threat. I never have trouble when I'm allowed to put diseases up and get a howling blast off before some mage or huntard starts with the aoe, as for dnd I don't trust it because it doesn't generate as much threat as a full rotation and cripples your ability to use said rotation.
For aoe trash I put diseases up, spread them. Then HB> BB SPAM>Rune Dump>HB while keeping diseases up of course, I start using single target threat spells such as OB when the wave starts to die down, I reserve dnd for big pulls, 4 or more mobs usually. You certainly don't need to waste 3 points into morbidy as frost like most death knight tanks do. You would be suprised how many horrible tanks can't seem to hold threat unless they can use DnD twice per wave. Just remember the golden rule of tanking, if dps die it's their own damn fault. So don't let them grief you and don't give in and resort to dnd spam to keep them alive throughout their mistakes, dnd spam doesn't work for anything you're not over geared for.
Yes it's true that most death knight tanks love DnD and use it on at least 85% of their trash pulls, but do you know what else is true? Most Death knights had little to no tanking experience before the class was released, most death knights aren't tanking end game content, or doing much of anything they aren't over geared for, but more importantly healers all over the internet are complaining that death knights are too hard to keep up as tanks and don't hold sustained threat on longer fights, meaning they end up having to spam heals on the whole raid, do you not see the irony?
Let's look at the down sides to using dnd as a bread and butter spell in end game content, you have to sacrifice 3 talent points, you don't generate as much tps as a dk using a full rotation due to lack of runes, and you will be frantically using all your runes to try and hold threat, leaving no room to use the defensive cool downs which death knight tanks rely on. The choice should be obviouse.