I like to stretch after warming up a bit on the bike. Stretching is most effective when your muscles are warmed up a bit. I always mean to stretch after the workout, but I rarely actually do =/

If I don't stretch enough, my muscles feel sluggish and unresponsive. Especially my quads. I also have a friend who damaged his hamstrings by riding almost every day without ever stretching them ...

I guess I figure, mild stretching can't hurt, and it's probably very good for you.