If and when I give an altitude and QNH I expect you to use this and not 1013,2. There´s usually a reason for this. In our sectors we have an instrument approach that starts at 6000ft QNH. We clear arrivals to another nearby airport to 7000ft QNH (the same one of course) instead of FL70 at pressures of 1013 or below to a) get these guys down on profile and b) give the QNH at a convenient time. We also have special flights like photo missions flying on local QNH that require other (descending) traffic setting local QNH as well to provide separation.