OK... here is my take on it, granted I got this information from article I read, but there is no use trying to say it any other way because it reads perfectly...
Stretching cannot be underrated in the scheme of resistance training. It improves range of motion by almost 20 percent immediately, even when flexibility has been minimal. It's important not to go into any workout cold, but in pre-workout mode, you should be doing lightweight sets of your first movement, much more than actually sitting down and stretching (warm-up). It is important to warm up using the movement you plan to start with, so that your joints will ease into the workout and blood will be driven into the joints being worked and muscles will warm slightly and engorge with blood flow.
Post-workouts are different, those should feature a full stretching routine. It is believed that a full-body routine is most beneficial no matter if you just trained biceps and back, or calves and abs, or your whole body. The point is to stretch joints, muscle bellies, muscle fascia, tendons, and ligaments on a regular basis, and usually always following workouts. Also, recovery improves with stretching, since extending and lengthening muscles as opposed to contracting them stimulates the release of lactic acid and toxins. That and 20+ ounces of water following a workout can minimize soreness tremendously, and help aid in faster recovery. Make time to do this at least three days a week, if not all the days you workout.
In the off-season, it should take the place of your cardio-time. By the time pre-contest diets and training roll around, your muscles will be more supple and muscle quality will have improved because you have broadened the range of motion. You may even be surprised at the kind of growth you get from adding stretching to your regime.
