Yes, I do stretching on ground and in the saddle, as I warm up the horse. It helps me to relax and my body to become more symmetrical while riding. I try to stretch all the main muscle groups and be sure that even my facial muscles are relaxed and supple, because tension in one part of the body means it is likely to be tense somewhere else, too.
Also, I believe, that if I am to ask my horse to be relaxed and supple, it would be unfair if I asked him that while I'm all tense and stiff myself. After all, we should help them in movements in balance, not get in their way just because our bodies aren't ready for the exercises we ask our horses to practice.
I have come a long way, to surrender my shadow to the shadow of my horse.