So! First off let me start with a disclaimer, my Death Knight is lvl 66 & im a terrible melee player, with that said..
Stamina, as a dps in raids is "worthless". Ignore it. It should never ever factor into your decision about gear.
Strength, as a plate wearing dps IS important, quite important in fact, but keep in mind for PvP you were likely gemming resilience or some combo of that, now you have those extra numbers to put somewhere else, pure strength gems being a very good option.
"Off stats", YES, do make a fairly large difference on a piece by piece basis and a HUGE difference in comparing an entire set of gear. In raids, wearing PvP gear is bad, because raids are all about optimizing your dps, resilience doesn't help your dps, so you have wasted stats. Wasted does not = optimized. However, in saying that, for 5man heroics, I wouldn't get too worried about wearing PvP gear until you get a proper PvE piece to replace it.
At the end of my rambling: Wear your PvP gear in 5man heroics, get valor points, buy & use valor gear (idk about DK's but for the classes i play, get your tier set first!). Also I do suggest you get justice gear, make sure its the 359 stuff, but yea, its worth it dude, 1) because it will increase your dps and 2) Raid Leaders/guild leaders would prefer to see you in PvE gear for raiding.
Best place for information on classes imo: The official wow forums; http://us.battle.net/wow/en/forum/1012662/
Check out the Frost PvE Guide 4.2