JW411-I believe your transponder transmits on 1013,25/2992 all of the time and not on altimeter setting.ATC computer works out passing alt using the difference on QNH and standard pressure on the day.If you mean an alt verbal readout for a crosscheck with ATC use standby altimeter which is usually set to QNH for terrain clearance .
I think when cleared to a level one should set 1013 right away,also depends on your rate of climb etc.
An opinion only.........