There's a few different reasons to check the calibration. If any of the sensors have been damaged or you suspect they may have been damaged. If any sensor has any type of play. As a preventative measure, some shops do this once a week, others once a month. Not all shops take care of their equipment equally, so I can't give you a blanket answer that covers all shops. Bottom line, if you question the readings, then it's a good idea to spend 10 minutes to check the calibration.
