I think the perspective in this topic is wrong. Being good is not knowing when you need to "use" tank stance.

By default, tanks are in tank stance. Being good is knowing how you can use your offensive stance.

You don't start in your DPS stance and figure out when you should be in your tank stance. You start in your tank stance during progression and should be actively analyzing damage intake and mechanics to figure out when it makes sense to swap to deliverance / sword-oath / grit-less.