With the older rotary Transponders the policy of turning the unit to STBY has merit but with modern keyboard Transponders the likleyhood of an incorrect code is slim and is easily rectified.
The problem with turning the Transponder unit to STBY is that TCAS protections are now lost which in my mind is not an ideal situation.
By all means turn it to STBY but just realise the repercussions whilst it is off and god forbid if it is not turned back on.