Stretching and warming down are two very different things. Stretching can be useful after cycling because you tend to hold a very fixed position, and can have quite an achy back, neck and shoulders after a ride. Stretching when you are warm (after exertion) is fine and useful.........don't do it beforehand, though. Warming down is just a gentle movement to help take lactic acid away from your muscles, to reduce the amount of stiffness you feel in them the following day. Hop off your bike and walk around the garden a couple of times. Don't anticipate marvelous results from any warm-down regime.