SORRY, I have to disagree, I have seen RF occur at mid and high range and not show up at idle. I have had it happen to me and I witnessed a pilot coming close to loosing a Patty due to vibration at mid range and not at idle. Muffler bolt-loose muffler pipe- IG box- Spark Plug wire worn and rubbing against the engine- throttle rod- even traced one down to a cracked crystal that would not show it's ugly head until the plane was in the air and the throttle was cut back or wide open.
Better to be safe than sorry and it don't take that much time to check it at all speeds.

My $.02