As an ATCO I'd expect you to set the relevant pressure (STD or QNH) as soon as I clear you to a FL or an altitude - at that point I'm not relying on vertical separation against other traffic anyway.
I am aware that a number of airline SOPs require you to wait until the TA/TL before doing so - also not generally a problem for me as per separation comment above. However...
A friend used to fly heavies for a certain airline, which mandated waiting until TA/TL to change the pressure setting - I remember him warning me about the very significant risk of a major level bust in the descent if the QNH was low, and was always cautious about descending them to 1000' above anyone else in that scenario.