The initial question seemed to be whether good airmanship dictated that turning the transponder to standby would prevent wrong squawks showing on radar. That seems sensible, especially when you think (as many of you don't seem to be) that we are talking about student pilots with twiddly knob transponders and an information overload. It is not as simple as you airliner types with two transponders and push buttons. It seems a good starting point from which you can progress when you start moving to push button or two transponders etc.
From an ATC point of view we have a system called Auto-DM which recognises the squawk assigned to a flight in the computer. Within certain parameters, if the computer sees that squawk it will activate the flight. If you have by chance selected the squawk in error then the computer will produce flight strips, estimates etc for a flight which is not yet airborne. The plan will then need to be cancelled and re-input. OK so the chances are small, but we are in the business of minimising chances of error, so if you can do that by selecting standby then what's stopping you?
I will certainly continue to teach my students to squawk standby, especially as some of them take 30 seconds or more to finish twiddling the buttons!
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"Take-off is optional, Landing is mandatory"