Why even go down that road? If the race wants to put "Podunk 5k" on its flyers, that's up to them, but the cert should read "Podunk 5.32 km" - the exact distance. The will eliminate any confusion when runners go to the search engine, too. Chances are the race name is going to be more grandiose, anyway - like "Pete's Perpetuallly Perambulating Podunk 5k."