The transponder must be set to GND on the ground. ALT in the air. Standby is pretty much only used in the hangar/times of maintenance. Part of the cert spec is it must automatically change from ALT to GND after landing and GND to ALT when taking off.
So no, pilots shouldn't be putting the transponder to standby unless there is a real reason to do so.
OK, that clarifies things a bit. I've been trying to find some info on this "GND" thing for a while. The ADS-D Decoding Guide http://adsb-decode-guide.readthedocs.io/en/latest/index.html
makes no mention of it. So I have two(ish) questions:
What input/sensor does it use for determining GND/ALT? I see a lot of aircraft that jump from GND to 25, 50, 75 feet while taxiing (and sometimes at the gate). Once in great while an aircraft will jump up to cruising altitude while at the gate (and at 0kts). Then there was one time I saw 60000ft at the gate! Clearly the later two are errors (the coding only allows for up to 50175 feet), but I don't see why the first example is reporting a (low) altitude when on the ground. Any thoughts?
Perhaps this is related: How is "GND/ALT" coded in the message? The altitude part can't just be 0 because that actually means -1000 feet. Again, the Decoding Guide doesn't specify.