I believe that the ICAO PANS-OPS method is to set the standard setting when cleared to a flight level, although I can't give you a reference until my next visit to the office. The FAA AIM doesn't state when to change, but states that the reported altimeter setting should be used when cruising below 18,000ft and 29.92 inches when cruising above 18,000ft (AIM 7-2-2).
BOAC....On the 744 we just press the button and STD is set, as I'm sure is the case on most modern aircraft. I'm not sure if this sets 1013mb or 1013.2mb, as the baro setting does not have decimal places. However, I make the difference between 1013 and 1013.2 nearer 5ft than 50ft.
Airclues