If you want to, you can do PNF style stretching to “trick” the nervous system reflexes into allowing greater range of motion (assuming for some reason you would care about ROM). But it´s not a very bright ideea to stretch a sore muscle. The pain and stiffness are essentially protective mechanisms to discourage you from lengthening a muscle outside it´s perceived range of safety. Better go with foam rolling post workout.

I really like to flex my legs after each set then a really good stretch once the workout is over

Always stretch AFTER the muscle is trained..else you will lose potential strength in the muscle....for legs do Sissy Squat stretches....they will stretch your entire quad....and follow that by straigh leg toe touches

Stretch after and I find foam rolling to be good to release lactic acid buildup after an intense workout.

The way it was explained to me is that muscle is somewhat like a rubber band. If it's cold you don't want to stretch it out but once it's warmed up (like after a workout) the stretching will allow the fibers to elongate and ultimately provides an environment less conducive to injury and more suited for nutrient uptake.