Unless it's during your individual show (or people behind are about to crash into you) I always let them stop - because they have white hind legs! And i'd rather like them to remain white.
If I was in a dr test or doing the trot bit or my individual show in hand then I'd keep them moving as they only last a few mins anyways (the tests I mean!) but if in the show they were all walking round or lined up then I would def let them stop.