Not sure if someone mentioned this or not, but a lot of it depends on your healers. I usually start off pushing my limits without my tank stance, and then turn it on if I notice the healers are having a hard time, OR if I notice things might be going down the wrong path (like if one of the healers die).

A classic example of this is in Susano, after the third Stormsplitter later on, one player will get locked and everyone will have to jump to the other side of the map. In most of the parties I have been in, this phase is always risky as hell to do without my tank stance. I don't know what it is, it kinda feels like my healers just suddenly forgot about me. Even though I have Rampart + Convalescence on, if I'm not on tank stance I just keep seeing my HP repeatedly drop, and I've even died a couple of times to this. -_-

So pretty much just drop Grit as soon as you feel you have enough enmity, and see how your healers react to it. This is how you learn what you are and aren't able to get away with, and you'll improve over time. But it will largely depend on how good your healers are. And if you have REALLY bad healers, dropping your tank stance will usually end up in you dying.