Cycling doesn't require your joints to go into their extreme ranges of motion, so I don't see the usefulness in static stretching. Most people do it because others have always done it. What seems to be more effective is dynamic movement (chicken dances, etc.
) such as leg kicks, bodyweight squats, actively flexing and extending the knee of one leg while you're standing on the other leg. Getting more blood flow, increasing internal temperature and decreasing tissue viscosity (from a proper warmup) seem more useful.
A study a few years ago showed that runners who engaged in static stretching did not prevent any more injury than those who didn't. Take that for what it's worth, but I stretch AFTER I ride (when I remember).