I usually do a couple sets light before i start training on a specefic bodypart, after you're finished your workout then you can stretch, When you work out, lactic acid builds up in your muscles. This often leads to muscle soreness and fatigue. Stretching after a workout helps to reduce muscle fatigue. When you stretch after a workout, your muscles are warm and you benefit from increased blood circulation.
Stretching will help your muscles to recover faster from a tough workout. Muscle soreness is one of the reasons that many people skip exercising. When you stretch, the likelihood is much higher that you will proceed with your next workout and prevent future injury.