You can do it before or after ITA.
Things to consider:
-How long you have to wait to get an appointment. If your medical center doesn't have any opening for the near future I will at least make an appointment soon.
-Your score. If you think you will get an ITA soon then there is no harm in at least making an appointment. If your score is low and you don't think you will get invited soon then hold off until the draw points are closer to yours.
-Medicals expire after 1 year.