It is generally recommended to leave the microwave oven door closed when it is not in use. Closing the door helps to keep the oven clean and prevent any accidental contact with the inside of the oven, which may still be hot if it was recently used. Additionally, leaving the door closed helps to maintain the integrity of the microwave's safety features and prevents any potential hazards. However, if your microwave has a ventilation feature, it is important to follow the manufacturer's instructions regarding proper ventilation and airflow.