While having this option checked off makes healing far less stessful, it is still easy to use damaging abilities with it off. Your casting 'cone' is about 45 degrees, so if you turn your camera with the target you'll land your spells no problem. However, if you need to turn very quickly (ie. the mob is running right next to you) you will want to overshoot the camera a bit, as latency will make the game think you are looking further back than you are.