ShyTorque
9 times out of 10 when I'm given a squawk, I'm also given a QNH. I was taught that reading back both comes before dialling in the new squawk. If I don't write both down, 9 times out of 10, I will have forgotten the squawk by the time I've read back the QNH, and certainly by the time I've reset the QNH! Others may have better short term memories and recall though. I have to say that modern transponders with buttons for the numbers are a huge improvement on ones with dials. Worst are the ones where you have to press a knob to select the digit, turn it for the digit required, press again for the next digit etc.
My point was that it's just as quick to dial in the transponder code while it's being passed as it is to write it down (my transponder has four rotary dials). Same with QNH, just set it directly on the altimeter.
It's often obvious on the radio who is fumbling with their pen before reading back the figures to ATC due to the delay in replying. As well as blocking a busy frequency, it's all extra time "eyes in" because you then have you back inside a second time to change the settings.