Situation's galore! Generally it depends on many things. Durining pulls I.E big pulls i keep it on all the time but during boss i do aggro combo and if im happy with my aggro lead and the incoming damage isnt too high then i turn it off and dps all the way. But if i notice a dps die or the healer needs to do extra work and turning tank stance on makes it easier for them then id turn it on.