In my airline our SOP is:
Above 3000ft AGL AND cleared to climb to a Flight Level, set STD (1013).
In the descent when cleared to an Altitude set the QNH given.
This is our SOP and having talked to some controllers once ATC have cleared you to a FL/Altitude, they expect you to be on the relevant pressure setting! Plus as someone has already said, if you start doing your own thing and waiting for transition level/altitude = level bust waiting to happen.