Stretching is very important.
The key is to be able to stretch lower than your race position such that when you are in your race position, there is no tension on your muscles. If there is tension (even slight tension) on your muscles, they don't fully recover between pedal strokes.
I like to think of our muscles as workers and if they are working, they aren't available to take a micro-rest to receive nutrients.
A great way to illustrate this is with a heart rate monitor. There is a point above which when your warmup pace might have you at 110BPM then if you go down too low (just a few inches) your heart rate will go up to 140BPM. This is because you aren't letting the muscles rest between pedal strokes.
I've even had it happen when I'm on the bike being held waiting to be pushed off for a flying 200 or Keirin. I'll use my muscles to stabilize myself which is bad. Then you launch off with not-100% recovered muscles. The key in that situation is to focus on letting your legs hang and all of the weight is on your crotch and allow the holder to keep you upright.