I would consider Desolation if you plan to have Reaping, you need either BS or Pestilence to convert your runes get the extra damage and tps from BS.
Necrosis and Dirge I didn't find as very usful talents in tanking, moving a point to vicious strikes will help with extra tps. If you find your taunts or disease spells missing consider virulence as well.
This is strickly my opinion however I am in the same boat as you, I'm looking for an Unholy build to use for AOE tanking and a solid single target tank build mostly considering blood. I have a thread a progress and hope for more responses: http://www.tankspot.com/forums/f175/...lf-builds.html
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I don't use DS all that much when tanking unholy, usually SS with IT and PS thrown in to refresh disease if the glyph doesn't proc. I throw in DS into the rotation on fights like General or any other healing intensive fight.
I always like to have the death rune talent in my builds, gives you some more flexibility and with the right timing of empower rune weapon you can get a good burst of threat with 3xSS or 3xDS with a Death coil or rune strike thrown in for good measure. I see desecration more of a DPS talent.