I generally agree with MAX whereby you set 1013 as soon as you are cleared to a flight level in the climb and QNH as soon as you are cleared to an altitude in descent. However companies I have worked for required the Standby to be left on QNH for the duration of the flight, as both MSA and driftdown are altitudes not FL's. The exception to this is if you are operating above FL290 where RVSM checks require all 3 to be set to 1013.