You might’ve noticed, back when I did my previous lrn2deathknight post, that all the attack rotations started off in a very similar fashion: Icy Touch, Plague Strike. Simple enough, yes? Honestly, these two are interchangeable for most of your career.
However, there is one very specific case in which you’ll want to guarantee to always cast them in the opposite order – Plague Strike, then Icy Touch. That case? When you put points into Glacier Rot in the Frost tree. If you put points into Glacier Rot, targets that already have diseases will take 10% more damage from Icy Touch if they have a disease on them – so use Plague Strike to attack and set up that first initial disease, then toss on an Icy Touch that benefits from Glacier Rot and adds Frost Fever, then go about your business – whether that be Scourge Striking, Heart Striking, or what-have-you.
Oh, and I can confirm – once you hit Northrend, you will more than likely want to move Death Strike in place of your Obliterate, if you haven’t already. Obliterates are great for plowing through Outland, but stuff – even equivalent-level stuff – hits a lot harder in Northrend, it seems.
Protip: get a Glyph of Death Strike as soon as you possibly can. (In my experience, you’ll want to find someone you know who can make it – DK glyphs are obscenely expensive compared to the other classes, at least on Feathermoon.) The healing from a glyphed Death Strike is pretty amazing for soloing, especially if you’re a tree like Blood that has more runic power than you know what to do with, or a Frost spec right before unleashing those Frost Strikes.
Real quickly, for Unholy, it is definitely preferable to use Plague Strike before Icy Touch in your DPS rotation as well, unless you have somehow specced deep Unholy without picking up Desecration, Ebon Plaguebringer, or Rage of Rivendare…in which case, you seriously need to re-evaluate your spec.