We'd really need more info to give you tips. Otherwise, you should just use a guide.
And the IT glyph has a very clear reasoning behind it. For one, DC is your hardest hitting attack, and SS is kind of a joke right now (as I understand it, two BSs do more, so the Death Rune talent is worthless).
Unholy is running a 10-second rotation (you basically repeat the same thing every 10 seconds, working in HoW into one of them). So, you'll IT every 10 seconds, leading to an additional DC every 40 seconds. That should be better than 20% more damage on PS, 40% on UB or anything else. And the optimal build refreshes diseases every 10 seconds, so Epidemic and the glyph of SS are both useless.
But, 2/3 glyphs are completely different between AoE/ST scenarios. As such, the IT one is only useful for bosses.
[EDIT]
And not taking Necrosis is ludicrous to me. I really advise you to go with
this build. If you don't have the 4-piece bonus from tier 9, shift a point from Dark Conviction to Bladed Armor.
Edited, Nov 21st 2009 2:36am by idiggory