Topic done to death, but in my personal experience...stretching before tennis, bad, makes my legs feel weak. Better to just do a light warm up.
Stretching after: critical to being able to walk comfortably on my 40 year old joints and muscles.
I try to do a shortened version of the infamous Rodney Yee's "yoga for flexibility" DVD I have. I basically run through about 6 basic poses in 5 to 10 minutes and it does a world of good for my stiffness and discomfort the next morning.
I also then do 2 of the "greatest stretches on earth" that I saw on the sportskool series on demand. One that resembles a runners stretch but where you put your elbow to the ground, and then the other where you are in a lunge position and twist while leaning back.
If I don't do this routine, I often have sciatic pain, arthritic feeling hips, and general malaise the next day. If I do the routine, I just have mild DOMS. So, that's what works for me. The only time I've ever pulled a muscle playing was when I did a lot of hamstring stretches before my match.