Stamina stacking isnt a great option. You will hit for shit, even with Vengeance as blood you will hit for shit. As frost, even worse. It isnt viable.
In terms of haste for frost, it isnt viable in pvp and to a "lesser" extent in pve (meaning it has some worth). Check out some recent DK guides before I make one myself. Straight str is the way to go, and the rationale is straightforward enough.
DST is great for unholy albeit its worth can be limited after >33-40% haste from gear + unholypres. For frost it isnt that amazing but hell, its still fucking great. I consider TT Better in some aspects, eg, AP over haste although I Still want a DST. The haste is for the rune regen and procs for those who dont know it, we dont have necrosis or BCB anymore, although they were somewhat weak talents. Pick up a DST anyways, but ID suggest for pvp running with a Trink and Berserkers Call if not human OR Naaru / Tsunami / Battlemaster and Berserkers Call if human.
Str in red,
str / resil in yellow,
spell pen > str/stm OR straight strength in Brutal gear. Haste isnt the best option for pvp as frost.
Im shortening my responses alot, but in short, Dont stack stm in general, haste isnt a frost priority in pvp, and pick up a DST anyways