Going to do a mini hijack here, but how much stretching, and what actual stretches, do you recommend? I was under the impression that bodyweight or light weight repetitions of the exercise you're doing would suffice. For example, when I squat I start out doing 2x5 with the bar, then 1x3 with 25lbs on each side, then 1x2 with 45lbs on each side, then 1x1 with 70 lbs on each side. After that I move on to the 3x5. Are you saying I should be doing actual stretches in addition to that? If so, do I do them before my workout, afterwards, or both?
Before a workout, the goal is to warm-up your muscles to get them ready to work. To accomplish this, some light cardio (running, rowing, jump rope), light bodyweight exercises (push-ups, pull-ups, air squats), lighter sets of the actual exercise you'll be doing (lighter squat sets as you wrote), and
dynamic stretches (windmills, leg swings, high knees, butt kicks) are useful. My typical warm-up is 500m of rowing, windmills, leg swings, air squats, push-ups, pull-ups, sit-ups, and supermans. If I'm doing heavy lifting, I'll also do a series of barbell exercises using a broom stick, including back squats, front squats, overhead squats, good mornings, overhead press, push jerk, cleans & snatches.
After the workout, when your muscles are nice and warm, is the best time to work on increasing your flexibility/mobility. The best tools for this are static & PNF stretches. Exactly which ones you should do depends on where you need work.
Having said all that, working with proper form on barbell exercises is, in and of itself, a very effective stretch. If you want to improve your squat flexibility, put some weight on the bar and squat. If you maintain proper form as much as possible within your range of mobility - post a form check video if you are unsure - then having the weight on your back will gradually stretch you out until you can do the full ROM. In fact, as is pointed out in the
active hip article I linked to earlier, in the squat, most people are limited by bad technique rather than flexibility, so just practicing the technique over and over again (with light to moderate weight) is a very worthwhile activity.