Thanks to bookofjoe, I have a reference to show for this tip in the latest edition of Cook’s Illustrated, one of the best returns for one’s money if you have an interest in food. Read the full post at bookofjoe for the entire experiment done by the magazine to verify this. They take a lot of care to see that their recommendations are not old wives’ tales. However, the summary is:
In sum: Don’t pull grapes from their stems before refrigeration. Simply discard any that show signs of rotting and hold off on rinsing until just before serving.