When should I drop tank stance?

[JonBigwood]

As you are asking in a dungeon, I assume it is a dungeon with unknown people that will not be repeated. You may be matched with very good players with top gear who do a lot of DPS and may steal your aggro unless they refrain themselves. Or with undergeared players who don't do their rotations at top efficiency that is unlikely that steal your aggro. At the same time they may be players that don't take avoidable damage with a healer that also does DPS, or players that take avoidable damage, don't know the place need a lot of heals, etc.

From my point of view, as a tank the priority would be not lose aggro, place the mobs and boss well so that the group is protected and they may do the best DPS, and avoid all the damage you can avoid. All while doing your best DPS. If you do all very well except changing to DPS stance and you have great DPS and healer players, all will go fantastic. You might add some extra DPS and icing to the cake if you go to DPS stance optimally but if not, all is fine. If you lose aggro and have to change stance back to tank, or use provoke etc. your rotation will be spoilt and your DPS lower. You might just need to use some aggro combos in DPS stance, though.

If you are matched to some not so top notch players or not so well geared, maybe you don't risk that much losing aggro, you can go to DPS stance but it is nearly sure your better DPS can't overcome the worse DPS of the other players, and some avoidable damage will happen, maybe deaths, occupied healer, etc.

When you know well a place, what is going to happen, where to place the boss and mobs, etc. you can focus much more in stance dancing, taking care of not losing aggro, doing your best DPS. In a boss maybe one aggro combo and then change to DPS stance, maybe wait for some time then change. Always looking that you are not near to lose aggro.

Also when you run a dungeon with people you know, have been with them there before, each time you go, you can fine tune everything bit by bit and make things go better. This includes stance changing. You can try to pass to DPS stance a bit earlier, see what happens and fine tune.

With unknown people in a one time and never repeat situation, it is not that important, just your personal adaptation to different groups, pulling more or less, passing earlier or later to DPS stance, etc.