A rather well run simulator session many years ago showed me that whenever you have the FD and an autothrottle on, you must follow the FD. Not following the FD may result in inappropriate A/T commands. The A/T will only perform as expected if it's in the correct mode for the task in hand. If you are not going to follow it, dump it. Why? On a non-precision approach, NAV or HDG with V/S or FPA work to give you the desired track, speed and V/S. When you disconnect the A/P, what mode will the A/T be in?