Stretching is highly recommended either before or after your workout but can really be done at any time. Corrective stretching is done before exercise but after a warm-up to lengthen chronically short muscles and help your body remain aligned properly. Relaxation stretching is done after exercise to lengthen muscles and help them return to their pre-workout length. Both types are good, should be regularly, should only be done when muscles are warm and should feel comfortable and painless.