All athletes are trained to stretch prior to competition to prevent injury but science say that it doesn't improve overall performance. Out of 23 articles examining the affect of stretching prior to performance, 22 articles say that this does not improve the ability of muscles to stay at a constant length, exert greater force, or cause the body to jump higher. These three attributes were studied because they are known factors in measuring elite performance. However, a few studies did show that regular stretching did improve performance.
This mean that stretching before and after training or workouts will be beneficial to the athlete when it is time to perform. Our professional trainers and athletes may know this, but it's important we practice good stretching techniques on the amateur level. If you are a track and field athlete and want to improve your running speed, there was only 1 article that showed regular stretching improved running performance compared to those who did not stretch regularly. Keep in mind, this study only tested short distance running (50-yard dash) and more research is needed for long distance running. Stretching before you perform is not a bad thing and as an athlete you should always do so. You will decrease your chances of being injured and also improve your running economy.
For more information on stretching and performance visit the Clinical Journal of Sports Medicine.