Interesting question - one I have not given much thought until you asked. It might make sense to end your singing performance with a softer song in the easy part of your range, but often singers like to end on a crescendo with a really big number. So what then?
In real life, most singers do some talking after ending their performance. So just talking could act as a cool down exercise for the vocal chords and other muscles involved in making sounds. But perhaps if you end your performance with a rousing number than requires you to really push the limits of your vocal delivery, and then plan to go into solitary confinement - perhaps you might consider singing softly to yourself to "cool down".
The reason it's important to cool down and stretch after intense physical activity such as running, cycling, rowing, etc. is that your heart rate has been increased and it is better to gradually bring down your heart beat than to suddenly stop activity. You have increased amounts of blood pumping to your large muscle groups and suddenly stopping those muscles while the blood supply continues to pump into them at an accelerated rate can contribute to cramps.
Also, you stretch after intense exercise to help rid the muscles of the build up of lactic acid that occurs during intense physical activity.
The same principals might apply to singing, but not to nearly as great of an extent. Although intense singing might elevate your heart rate, it's not as dramatic as running or other aerobic type activities. And there is probably very little (if any) build up of lactic acid during normal singing.
So I'm guessing that the reason cool down after singing is not something that is emphasized or widely practiced in choral groups or singing instruction, is that for most singers, the act of talking after the performance probably provides all the cool down that is needed in most cases.
But as a precaution, it certainly would not hurt to do some quiet singing or humming if you end your performance with a challenging piece that requires maximum exertion of your singing mechanisms. Especially if you have no one to talk to afterwards.