The only time I'd feel comfortable dropping tank stance is during boss fights; on trash pulls I need the extra eHP/enmity to keep everything focused on me, especially if my DPS are SMN/BLM.

Of course, it's not going to break the bank if you just sit in tank stance for an entire dungeon. Sure, you're not putting out TOP DEEPS doing that, but if you're not comfortable sweating a pubby healer with DPS stance, don't worry about it. Nobody's going to withhold a commendation because you focused on tank stance.