This is the priority listed in simulationcraft currently:
(Note: I took out some lines that were for PoF/ERW with trinkets/potion, and unholy blight)
And the following is my understanding of it:
1. Plague Leech if there are less than 3 sec on either Blood Plague or Frost Fever. In practice this becomes a matter of keeping an eye on Blood Plague since we're constantly refreshing Frost Fever.
2. Outbreak if it's off CD and diseases need to be refreshed.
3. Soul Reaper sub 35%
4. Howling Blast/Plague Strike to make sure diseases are up.
5. Frost Strike with KM up
6. Howling Blast with rime (freezing fog) proc
7. Frost Strike when 76 RP or higher (to avoid capping)
8. Obliterate IF KM procs and you don't have RP for Frost Strike OR if you have 2 unholy runes.
9. Howling Blast if you have 2 frost or 2 death runes.
10. Obliterate when there's atleast 1 unholy rune (not death).
11. Blood tap. This is when you have no runes for any of the above spells (HB, Obliterate), you're under 76 RP, but you don't want to Frost Strike because you don't have a KM proc. If you've done the rotation correctly up to this point, in my experience you should be turning a used up unholy rune into a death rune.
12. Death and Decay if you have no other options, and have that one pesky unholy rune. This rarely happens and I don't think I've run into a situation where I've had to DnD single target on a boss.
13. Horn of Winter to get RP.
14. Frost Strike with no KM procs.
15. Plague Leech if you have absolutely no resources.
16. ERW if none of the above are available, though I personally just wait a sec to get a rune and save ERW for Trinkets/Potion.
Might seem like a lot to process but once you get on a dummy and practice a bit it gets easier.
Basically the tl;dr is with diseases up, Frost Strike with KM is your highest priority (other than soul reaper sub 35%), and you use Obliterate to get rid of unholy runes or use a KM proc when you don't have the RP for Frost Strike. Other than that Howling Blast, use Blood Tap when you have no runes up, and Plague Leech as diseases are falling off for a free rune. Someone correct me if I'm wrong.