I have a JR X378 TX. I really DO NOT believe that the reason from the need of sending the TX to a legal service to change the crystal is related to any tunning to be done for optimal performance.
These RX uses IF (intermediate frequency) detection based on the chystal frequency, that means the Chystal frequency(local oscilator) interacts with the desired "on the air" frequency creating a seconday frequency, the IF. The IF is constant, what means there is no tune to be done in the RX. The "On the air" frequency is the one transmitted by the TX. The TX range of frequencies is very narrow band. For low 72Mhz band the start frequency is 72.010Mhz and high end at 72.49Mhz. He high 72Mhz band starts at 72.510Mhz and ends at 72.990Mhz.
I belive the performance wouldn't change if you use another channel. I think the real reason for sending your TX to change the Crystal is for someone having the control that you are changing it to something inside the allowed band. In this case, there's nothing to do other than send it to then.

Nilo