I stretch various leg parts at random times but nothing related to the timing of my regular exercise or bike riding. It just feels good sometimes. My pre workout stretching is done as part of my workout, starting my rides a little slow and gradually building up. It's actually kind of funny, at about the 3 minute point I am thinking what the hell, this sucks, no way I can go 40 miles but at the 7 minute point that wears off as my HR builds and at 10-15 minutes I am feeling fresh and going full bore. It's the same when I do strength training. I have to do at least 15 minutes of gradually increasing cardio first. Call it what you want but that's my "stretching".