Oh mother or god I hate you people.

I tank because I want to tank and gear my tank.
I dps because I want to dps and gear my dps.

Respect the people who have been running Hour of Twilight 472917384927 times to get the freaking tanking shoulders after using the same blue justice point shoulders for over a *freaking* year.

If you know the group and they're happy to help you make a set, sure.
If you're pugging, ask before rolling off-spec.

No doubt, as you have predicted, your post will turn grey. If you want tank gear, queue as the tank. Doing anything other than what Olivia has described when tank gear drops makes you a complete and total asshat.

Honestly you don't even NEED to ask. If you're anti-social like me, you can just wait for their roll. If it's Need, then you should roll Greed. If it's Greed, then go for it. (Sure is funny people that hate humanity play MMOs...)

You sir, are dumb. The down-voting should reinforce my comment. The fact that you knew you were going to be down-voted shows that you know that you are dumb.

As a personal rle of thumb if im playing my DK tank and dps gear drops that will be an upgrade for me, wait for all the plate dpsers to roll. If even one of them rolls need I roll greed but if all roll greed than i take it as they don't need it and i am free to need them. When a tank piece of gear drops that I roll greed it any plate dpser or holy paladin may feel free to roll need on it.

@Ankun: Please don't talk about the plate tank shoulders of Archbishop I'm still running around in my T11 shoulders cause those are the best I got and the so call "Pauldrons of the Dragonblight" must be a myth cause I have run that place dozens of times and never seen them.

I'll agree with the requirement that dps should be careful about needing on tank gear, but because of inequities in queue times, I don't believe it should work the other way around:

If I'm tanking and I need that plate dps gear for my dps spec, you bet I'll be needing on it, without remorse or sympathy. Every plate-wearer with a dps spec can off-spec into a tank spec for an insta-queue and almost double his/her gear acquisition rate--and reduce queue times for everybody else by his/her personal increase to the tanking population. It's totally a win-win all around. And a rule that says that "tanks shouldn't roll on dps gear" tends to encourage dps to queue as just dps and not both tank and dps: exactly the opposite of the behavior we want to be encouraging.

I usually enjoy playing the dps spec more than the tank or healer spec. But if I queue as both tank[ or healer] and dps to reduce my (AND YOUR) queue time and the Dungeon Finder chooses me as tank[heals] instead of dps, you bet I'm rolling on that dps gear if my other, preferred spec needs it.

Please, let me know your main(s) and server so I can make sure to make your life a royal nightmare if I ever end up with you DPSing in a random dungeon that I'm tanking.

I don't really agree with this position, but I'm not completely sure I agree with the remark in this article about far more dps gear dropping. Tanks gear up faster in instances and raids than any other role - the majority of the time they also don't run into people needing on gear they would use (or I haven't personally when I tank, and I haven't seen many instances of this happening when someone else is tanking, either).