As a Pilot and an ATCO:
When departing and cleared to a flight level, I will set the main altimeter to 1013 when passing MSA and leave the standby on the aerodrome QNH.
I expect a similar approach from the traffic I control. If you are cleared to a flight level, it should be terrain safe and it is the FLIGHT LEVEL, not the ALTITUDE which is providing your separation against the traffic above which is, presumably, also operating on 1013. If cleared to a FL and not asked for intermediate altitude reports, set 1013 as soon as you are above MSA.
In times of low QNH, a delay in setting 1013 until TA can end in tears.
Consider this:
High performance biz jet departing on a QNH of 983mb. Initial SID level is 4000ft, and the aircraft is subsequently cleared to FL 80 against traffic at FL90. TA is 6000ft. The crew, with the a/c climbing at 3000fpm, delay resetting altimeters until passing 6000ft.
6000ft on a QNH of 983 mb is approx FL 69. By the time the crew have set the altimeter to 1013 passing 6000ft at 3000fpm, the aircraft will be passing FL75 or more, with 6 seconds or less for the autopilot to capture the new level and start the level off. Chances of a level bust or worse very high.
There have been more than a few Airproxes in just this scenario.
