Very good question, But there is no real answer. Some pilots cut the throttle on the top of every loop, and soemlike to fly it full throttle.
there are some arguments that will support both. I don't like to see people do either. If you shut the engine down at the exact top of the loop more
than likely you will flattten the top of it. Depending on the wind conditions, speed of plane and size of loop you will adjust when and if you cut the
throttle at all. A good rule of thumb is in an inside loop carry power over the top to about 25 degrees and then gently throttle back. as the plane coasts
around bottom start to smoothly advance throttle back to full. as airspeed increases the plane s elevator will seem more sensitive and less will be
needed to keep the radius contant. I rarely drop the throttle to idle, I prefer to keep some power throughout the maneuver.