RAPA
If you wait for transition altitude you will eventually have a level bust !
If the stop was FL50 then on a low QNH day if you wait for transition you will bust the level. (e.g Birmingham). Particulary on higher performance jets.
The only bomb proof way is by setting standard (on your primary altimeters) as soon as you are cleared to a flight level. On a low level off that could be after or even before flap acceleration depending on aircraft type.
You dont wait for ATC you just set it !