Disclaimer: My guild has not yet cleared ICC-25 on heroic modes yet. We are 9/12 and thus have not attempted Lich King on heroic yet. However, I've dps'd all of the fights (with the exception of LK of course) and feel I can speak to how to optimize your play as dps DK even on Putri and Sindragosa who haven't fallen over for us yet.
I'll also just do this one post for all of them, since most of the suggestions for each fight are still valid.
Marrowgar: Actually, if you weren't doing it before, on normal mode with a full 264-geared group, you can stay in for Bone Storm, using IBF as needed. The damage isn't that bad, and healers can easily keep you up through it.
For heroic, however, I doubt you'll be able to stay in even in full 277 gear. It hits HARD. So, stay out. Mostly, stay alive. Second, dps the people in bone spikes since those continue during bone storm. Third, IF and ONLY if you have both health and no one needs to be broken out of spikes, you may Icy Touch/Death Coil Marrowgar. More importantly, know when the storm is going to end and start cheating back towards him so you can jump on him as soon as he stops.
Lady Deathwhisper: we found this a little challenging. Of course, the mind controls and adds continue throughout the fight.
In P1 we pull all the adds on top of the boss for aoe to get them down fast. You may be asked to help death grip or control adherents. I actually have an assigned adherent that I DG to the stage. Positioning can be a bit tricky with the pillars - the LOS can work both for and against you. Once I get them on stage I usually mind freeze the next cast and then drag them to the perfect spot if they weren't already. If you're on add duty, you'll be gaining aggro frequently, so make liberal use of interrupts and ams to ensure you don't die. You can aoe, but be careful to not kill your mind controlled fellows. Interesting finding: when you're on add duty, you don't seem to ever be picked for mind control. I've done dozens of LDW attempts/kills on heroic now, and never been picked once.
In P2 if you're on the boss/add duty I recommend speccing deep enough into frost for free mind freezes regardless of main spec because you DON'T want to miss and interrupt. I stay on add duty, so I continue to control adherents and keep them in the 'cleave team' for maximizing kill time. Pestilence is definitely your friend throughout the fight, but death and decay rarely is, since you don't want to kill mind-controlled fellows and there are rarely enough adds standing in one place to make use of it. As unholy, I usually leave my ghoul on the boss, but if we're falling behind on adds, I'll pull him off for a little extra oomf.
By the way, a simple /petattack macro is your friend. I don't believe I've mentioned it before, but on fights like this where you're switching targets often, it's really nice to just have a keybind to send your pet to your target.
Gunship. Really? Just kill stuff.
Saurfang: I don't do anything different on this fight. If you're unholy, be sure to pestilence the adds asap - they need to go down fast and giving your ranged more dps on the beasts really helps. If your raid leader says not to - they're wrong.
Rotface: Just be careful when running around to avoid people with vile gas. DBM is ok for some things, but AVR/AVRE really shines on this encounter, showing you the danger zone for those that are vile and also where the ooze explosion is going to land. However, dps optimization is the same as before.
Festergut: Just make sure to avoid getting goo. If you do, the haste debuff will KILL your dps. In a pinch, you can still easily survive a pungent blight without getting all three inoculations with IBF/AMS, but try to ensure you get all 3 anyway - save your healer's heart, and mana.
Putricide: A wonderfully challenging fight! If you enjoyed the fight in T9 gear, the heroic version is a nice challenge in 264 gear, too! A few differences here. I pop my army at pull but not my gargoyle. Why? Because the crucial part of the fight is the phase transitions where you get both a green and an orange ooze. You'll need your burst to get your assigned ooze/gas cloud down quickly (and yes, I know the encounter assigns it for you). Other than that, it may take some ingenuity on your part to ensure you don't die because there's damage flying around all over the place.
Blood Prince Council: Blizz laid the hate on for melee on this fight, heh. You'll get a DBM warning when you get to 6 stacks of shadow prison, but it won't be very dangerous until around 10. On last night's kill, I got to 19 and survived. AMS is your friend here. If you must run, do so under cover of your shield. If you don't have a shield, don't run. The only exception to this is empowered vortex. Even if it kills you, don't blow up a healer. Run, and die. :) But really, it's fairly easy to get the stacks to fall off when you're on a boss, and so when you switch targets you can run all the way across the room safely.
Incidentally, my fellow DK is a big advocate of run speed enchants on boots. I'm not. I'm 32 attack power all the way, but he swears by it. This fight and Marrowgar are two where I can see the point.
Blood Queen Lana'thel: Nothing really different here. Just moar dots (dps). During her air phase, it's even more important that you don't blow up squishies, and you might as well save your ams to ensure you don't die.
Valithria: same ol same ol. Seriously, on our kill last night someone commented at the end: "that was heroic?" Once your healers know how to travel as a pack in the portal, the fight goes very quickly. I don't care about dps on this fight at all, and just focus on doing things like keeping the mages from casting their frostbolt volley.
Sindragosa: There is zero room for error on tombs. Without AMS, you will be killed by the frost bombs. I initially was worried about running out after her 'death grip,' but I found AMS still adequately protects you. I still don't wear any frost resistance armor. Honestly, the fight requires clean execution from every person in the raid, but it otherwise doesn't seem to bad to me.
And then there was one. He'll have to wait for later. :)