I always stretch for five minutes before and after a ride, using yoga positions to stretch hamstrings, quads, etc. I read somewhere (I think in Runner's World) that stretching immediately after strenuous excercise releases (dissipates) lactic acid which has built up in the muscles. Lactic acid, as I understand it, causes soreness and stiffness on the day following excercise. At age 64, this seems to work for me. Stretching is a necessity!