In my jet it doesn't matter that much - press "STD" on the way up, if you don't, then an amber thing happens.
ICAO DOC 8168 OPS/611, (Aircraft Operations) Vol 1, Flight Procedures, (5th edition 2006) states in Ch1 (Introduction to Altimeter Setting Procedures):
"1.1(b) In flight above the transition altitude, the vertical position of an aircraft is expressed in terms of flight level, which are surfaces of constant pressure based on an altimeter setting of 1013.2 hPa."
I confess I am surprised by the absence of any mention here of 29.92in.
It further notes, however:
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"1.2 This method provides flexibility to accommodate variations in local procedures without compromising the fundamental principles"