kit : during descent ,it's not so important what pressure you've selected, most important it's when you level off. And if you level off with incorrect altimeter setting, the transponder will transmit 1013.25 ,the ATC computer will transform this with proper altimeter setting and the ATC will ask you if you forgot to change altimeter.
If ,by any chance you're instructed to stop descent at FL XX,then you should change again to STD. If you forgot to do so,the ATC will again prompt you to do it.
I have visited one ATC facility ,and I've been amazed of the amount of new,computerized technology they have there. They improved a lot since primary radar....