The only time I would use the hyphens would be in a playful, somewhat elaborate construction, for example:
You know how it is at the Smith house. Ten children, three or four of them screaming at any given moment, and mother and father dishing out dinner, approval, and smacks alike with a admirably democratic first-come-first-served policy.
But for a more formal notice:
As of May 1, employees will no longer be able to call ahead to the cafeteria to reserve catfish plates on Fridays. They will be available strictly on a "first come, first served" basis.