I'm assuming you've looked at the generally accepted 'BIS' (best in slot lists) posted on Elitist Jerks (if not, check them out here - Frost and Unholy).
So, now you know what to bid on, right?
Actually, it's not that simple.
While the systems vary, almost any loot rules in any raiding guild are built around one primary principle; fairly distribute loot. DUH! Which means, you won't get every single melee/plate dps item that drops. DUH, again! Which means, you'll have to pick and choose what to bid on, prioritizing how best to improve your overall performance as a dps DK. DUH, thrice!
Still, let's think about this for a moment.
What should you do when that lovely shiny purp drops from Magmaw? Bid? Or not?
And how should I spend my Valor Points?
Where does all the other stuff (rep, professions, boe etc) fit in?
I'll share a few ideas, but I'm sure there will be as many different approaches to gear decisions as there are raiding DKs. I DO have a few principles/guidelines that I hope are fairly clear and will make my own (if nobody else's) bidding decisions easier and more efficient/effective.
1. Melee/Plate DPS gear of the same tier is more similar than it is different due to the wonders of reforging. While some pieces are clearly better than others, even the presence of a 'bad' stat isn't as bad as it seems, since you can often reforge much of it away. Example: I bought the Hyjal exalted tank cloak, but reforged dodge to hit. It was a big upgrade over my 333 blue cloak.
2. Think long term. When you have the luxury, pick items that will last you longer.
3. Above all, UPGRADE FIRST, then go for BIS.
Last night on Throne of the Four Winds (or, as we call it, ...Of the Monkey Raid) a plate belt dropped. The mystery stats were haste and crit - actually pretty good! My fellow dps DK bid on it, but I did not.
Why not? It would have been a very good item, almost BIS. I didn't bid on it, though, because I already have the Hyjal exalted belt. If I were to have bid on it, and won, I would have lost points, or priority, or my list position, or whatever it is that my guild uses to determine who gets the next epic. And if a nice set of bracers dropped next, I wouldn't be able to get them, even though my 346 blues aren't looking so impressive any more.
Bottom line is; don't bid on a 359 upgrade when you already have a 359 item, yet. Once you have full or nearly-full 359, feel free to bid on BIS items. But for now, while we're gearing up, take items that upgrade blue or green items first.
What about valor?
You can grab the cheaper items first, but they won't give you as much raw stat jumps as a more expensive item does (e.g., the gloves would be a 22 str upgrade while the chest would be a 42 str upgrade). Also, according to #2 above, if I buy a cloak or boots or some such, I could very well be upgrading them soon, while other items (the ring and relic) may last you well into hard modes or beyond. It's actually a fairly tough decision. For me, though, while I find the chest and helm intruiging, I'll probably go with the following.
So here's my plan:
1. Bid only on upgrades from blue 346 gear to epic 359 gear, saving points for bigger upgrades
2. Get all the rep/crafting gear I can. I have the Ramkahen neck which should last until hard modes. I have the Hyjal belt AND back (I reforged the dodge to hit - it will be replaced but is a decent filler for now), and so on. I won't bid on upgrades for my rep/crafted epics until the other slots are upgraded. Sure, Ill hope for a Scirrocco or Zin'roc from archeology, but I'm not holding my breath. I WILL bid on weapon upgrades.
3. Valor: buy a) relic first b) ring second since they both last deep into Tier 11 content. Next, buy the gloves because they're cheap and allow me to get 2-piece Tier bonus sooner rather than later. Dps DK set bonuses are tasty. Next, spend valor on non-set pieces that I don't have an epic in that slot, and then finally on any set pieces I don't have yet.
Well, I've probably gone on long enough about this, but it IS a fairly important process, after all.