This is a question sports physiologists are examining with greater interest, especially since an Australian study published in the American College of Sports Medicine's Journal (32:271-277. 2001) showed that stretching before exercise does not appear to prevent injuries.
The practice of stretching before and after physical activity is based on tradition, common sense and practical experience, but now exercise scientists are trying to prove the benefits. It's well documented that stretching plays a role in rehabilitation from injury. It is also widely accepted that stretching improves athletic performance by increasing range of motion. With this information it seems to reason that greater range of motion means you are less likely to become injured by overextending your body while competing or working out.
Another school of thought holds that since you warm-up before you stretch, perhaps it's the warm-up that provides the benefit and not the stretching. However, as the research evolves, no conditioning specialist is backing away from the idea that flexibility is one of the key measures of athletic fitness (along with strength and cardiovascular endurance).
If you are comfortable stretching and feel better when training from doing so then keep it up. If you don't stretch and are doing fine I see no real compelling reason to start. I believe in proper warm-up before training and that usually involves several sets of your starting exercise with a lighter but progressively increasing weight.
