it's best to know your tank,and how the other coccupants affect feeding.corys are diurnal so the time is always right for them,but if other fish in the tank hored the food or are much more aggresive,they the corys won't ever get a chance to eat.
also,while time isn't critical to most corys sometimes it's better to feed at night if the fish are shy or skittish,but those conditions generaly reflect that something else is wrong in a tank.