I feed my horse grain anytime im there, I might get attacked for this, but when im tacking up I sometimes let her eat it then too. She's not one of those colicky horses though, I could never do that with my old horse.
The best time to feed a horse supplements/feed is when he's at a normal heart rate/breath rate, and when he has hay in his stomach. Having forage in their systems before any grain makes it easier on them. Time of day doesn't particularly matter as long as you keep it consistent and make any changes minimally and gradually.
Is this rule void when the grain in question is alfalfa pellets or beet pulp or timothy cubes and such? And the supplements, of course.