Changing Transponder code.(to ATC & Pilots)
 

When changing transponder code ( on a transponder panel that does not have a key pad ) do you first put the transponder panel to STBY, change code, then back to ON, or just change the code while it is on.
Because leaving it on, will mean as you change the code, digit by digit, you will pass through other codes!!

I was taught, and taught others in GA to go first to STBY, change code the back to ON.
But as I have seen in commercial flying on jets, this practice is not done.
It is always left on.
The only reason I can think of, is that you will momentarily turn off the TCAS during this procedure, then maybe forget to turn it on again!!

Feedback from ATC would also be appreciated. Does this practice cause you any problem on your radar screens?