Is it recommended to have an ADS-B transponder in ALT mode on the ground?
Yes, it's in the the AIM 4-1-20:


  3. Transponder and ADS-B operations on the
  ground. Civil and military aircraft should operate with the transponder in
  the altitude reporting mode (consult the aircraft’s flight manual to
  determine the specific transponder position to enable altitude
  reporting) and ADS-B Out transmissions enabled (if equipped) at all
  airports, any time the aircraft is positioned on any portion of an
  airport movement area. This includes all defined taxiways and runways.