I agree with Randy that it is not required. 9004 is for guidance only - not auditing.
It may be a good idea in some situations, like safety. For example, if you have a respirator program in case of emergency, it would be good to periodically review the training with the personnel, since they don't do it every day (might already be a OSHA or similar requirement). If you trained someone to assemble a part they make 500 of every day, there is not much point in refresher training since they already know what they are doing and do it many times a day.
Tom