It may depend on your definition of "stretching," but over the years, stretching has proven to be a lot less important for an athlete than was once thought. Certainly there are mixed opinions, but for the most part, studies show that traditional, static stretching (stretch and hold) actually hurts performance and increases injury. If you still feel it is necessary to stretch, simply for the sake of flexibility or comfort, here are a few guidelines to follow:
1. "Warm up" any muscles before stretching them (Example: Go for a short, slow jog for a few minutes before beginning your stretches. Before stretching shoulders, you may consider slowly swinging arms around a little bit to get the blood flowing to those muscles. Before stretching your quads (thighs), you may want to do several squats...etc.).
2. Never hold a stretch for over 30 seconds. Studies show that prolonged stretching (up to a minute or longer) causes more harm than good. 10 seconds is probably sufficient.
3. Don't overstretch. You should not feel pain, just stretch the muscle until you can feel it pulling a bit.
Flexibility is a good thing, and there are other methods one can use to increase flexibility besides the traditional static stretching. But stretching in hopes that it will prevent injury or increase performance has simply not been proven to be affective.