I would think it is more likely not the switch. Most of the time, when the ignition key will not turn; it is caused by either 1> the car having rolled and then pressing on the transmission parking pawl (which can be prevented by ALWAYS setting the parking brake BEFORE moving the shift lever into Park, and not releasing the brake until AFTER the engine is started) and/or
2> the steering wheel being moved when the ignition key is not turned on (this can lock the wheel; which sometimes then makes it difficult to turn the key in the ignition). The way to prevent that is to not turn the wheel unless the key is turned on.
I hope this helps!!!