The scheduled transmit/receive operations are designed to start at the exact time that you specify and RAIL accounts for any transition times and ramp times it knows about. In BLE, RAIL also has Rx and Tx chain delay values that make these times very accurate. In proprietary PHYs RAIL do not have the Rx and Tx chain delays so there may be some small error, but the timings should be pretty close.
Also, RAIL is only taking the idleToTx time for transmit and idleToRx time for receive into account. If the radio happens to be in a state other than idle, it will be subjected to the rxToTx or txToRx times which could be different and cause a scheduled operation to start at the wrong time. So, for now, those numbers just need to be set the same to ensure proper behavior.