What about airports like LHR where the local rules specify that "After parking, the Mode A code 2000 must be set before selecting OFF or SBY."
Assuming (the documentation is really vague on this, as already mentioned) the A320 transponder goes automatically to SBY at "ATSU in" or parking brake/engine shutdown/whatever, does it make a difference to LHR ATC whether you enter 2000 before that event or afterwards? ("before" requires to change the code when approaching the gate, which is probably not the best time to play with the transponder).