Stillin1 and Chilli:
Sorry, guys, I think you are a bit out of order suggesting that when a PPL follows instructors' advice, he is following an old wives tale. If you don't trust your instructor, who do you trust?
Of course there is bad advice and there is good advice and we should be prepared to learn from our experience and the experience and knowledge of others. I'm trying to do this from this thread.
I'm still confused as to the correct procedure so I looked up changing tx codes in MATS Pt 1 and the Air Pilot.
I could not find anything relevant in MATS.
The Air Pilot is not specific. ENR 1.6.2 covers SSR operating procedures but does not state that standby should or should not be used when changing code. It does say that the transponder should not be switched off or the code changed unless instructed by ATC. It also says that 'pilots are warned of the need for caution when selecting code 7000 cecause of the proximity of special codes...' These two points suggest that we should change code without switching to standby. [sorry, I couldn't pull the exact quotes out of the pdf file]
I did not do a very exhaustive search of the above publications.
So, I'm afraid I'm still not sure whether or not to switch to standby.