It hasn't changed much since the days of when all you had was toggling Defiance on/off. You should be stance dancing in a way that's appropriate to the encounter you're in and your gear level. Switch to Deliverance during periods of low, controlled damage, and switch to Defiance during periods of high or spike damage. As your gear improves, you can play it more dangerously in at times where you otherwise would have switched back to Defiance immediately.