I never instruct clients stretch when cold. My warm ups often include dynamic flexibility drills, and joint mobility drills, but never static stretching.
There are cases where stretching before an exercise is a good idea. If a client has difficulty with ROM in a certain movement, I incorporate static stretches after the warm up, but before the exercise in question. I only stretch the tight muscles that prevent the full range of motion, and it usually goes a long way in improving form.
After exercise sessions are over, I do use static stretching for a cool down.