Mr Robinson,
Just a couple of comments about your post. Your explanation is right on the money, only on a taildragger it will cause a need for right rudder, not left if the descending blade is on the right side as most are.
The airflow through the prop changes with angle of attack on the wing so P factor exists throughout the flight envelope and is at its max when in a high power/low airspeed situation (high angle of attack). Since models can fly at considerable angle of attack there is the potential for much P factor in flight. As you pointed out its not a big player in normal flight as angle of attack is usually small when crusin'. But its very noticable on climbout after takeoff or when doing a go-around when right rudder is not used. The vast majority of model fliers don't apply right rudder on the climb and results in the model pulling to the left.
Paul