When is it appropriate to use rudder? Well let me count the ways.

1. During the takeoff roll, it is the only way you can hold the model straight.
2. Whenever you use aileron to make a turn you should also use some rudder to keep the turn coordinated. On full size airplanes you always want to keep the ball centered in a turn. If you do the plane is not likely to do something that will surprise you. (Read that, snap spin)
3. Any time you pull the nose up at greater than 45 degrees a little right rudder will counteract the yaw to the left.
4. Of course there is always knife-edge flight. That’s a given.
5. To fly a perfect roll, either slow or four point, you should use a bit of rudder when at knife-edge.
6. Any aerobatic maneuvers that require snap or spin inputs.
7. On final for landing with a crosswind. Use the rudder to hold the nose into the wind and maintain alignment with the centerline.
8. Last but not least, taxi back to the pits.

Other than the situations listed above, I never use the rudder. Just fly with the right stick.