Correct me if I am wrong but in AUTO mode, when on ground, the transponder is as good as being on standby. It gets the signals from LGCIU and if the ATC wants you to turn the transponder on it should be set to ON position.
So after landing does it really matter whether it is set to assigned code or 2000 (of course assuming ATC has not asked for it to be ON on the ground, as a good practice though I set it to 2000 after we park and when I put the transponder to STBY)?