A warm-up period before any workout is very important. It helps improve the flexibility of the muscles to prevent injury. A general warm-up should consist of 5 to 10 min of riding a treadmill , elliptical or stationary bike. You'll be increasing your heart rate, blood flow, deep muscle temperature and respiration rate which readies you for your workout.
Stretching should be done following a warm-up and or workout, to increase your range of motion and decrease the likelihood of injury. The increased body temperature increases the elasticity of muscle tissue (collagen). You should stretch every major muscle group in your body and hold each stretch for 20 to 30 seconds.
I always tell my clients that stretching is just as important as any other aspect of your workout routine and the only way to reach your peak physical performance.