You do as the SOP says and the SOP probably says this-
Change to 1013 when passing TA when climbing (then you know where you are with regard to MSA in case of an emergency) and all three at the same time.
Change to QNH as soon as you're cleared to an altitude when descending then the autopilot will (should) level out at the correct place and not infringe any MSA's.
Having them set to different settings as a reminder is BS because the autopilot will tell you if it has a problem deciding who has the correct setting and they'll have to be set the same anyway.
If you need to remember something WRITE IT DOWN!