General rules imo for tanking in dungeons:
Trash packs: Always tank stance. There are some fringe cases for DPS stance, but generally only if you can only pull one pack or whatever, or you're vastly overgeared(dungeons that aren't item level synced at old caps for example, especially when trash didn't hurt). Rotate cooldowns, spam aoes.
Boss: Start in tank stance, secure threat(this depends on the party composition and how geared and/or retarded your DPS are, it can be nothing with smart people+ninja to having to do 2 full threat combos in tank stance at the start with retards who don't use threat moves with good gear). Then switch to DPS. Make sure to cooldown tank busters and keep some cooldowns up the rest of the time. Don't do this on certain hard hitting bosses if your gear is shit, especially in leveling dungeons, these are the worst compared to cap dungeons because of the defense formula fluctuating rapidly, or if your healer looks vastly undergeared/retarded or your DPS are eating all his attention by stepping into every fucking thing. But generally it's safe.
That's for pubs, if you play with people you know you can definitely do better, but for pubs I restrict myself to that, and I adjust based on what I see is happening around.
Also obviously due to how warrior works, do what Fennin said before you have Fel Cleave, you don't really want to "no stance" content before you have Deliverance and you don't want to sit on full rage either once you do but don't have Fel Cleave yet.