I think its one of those rules that is a good guideline to follow. I don't think it could ever be a hard and fast rule though. This thread was a great reminder of something to think about. Not often do I think about an allergic reaction to something as common as MTG.
Although it's true that reactions can occur at any time of day, the OP was just pointing out that reactions are less likely to be caught quickly if they were to happen at night. Usually there is someone around during the day at a barn or at least we're feeding during the day for those of us that keep horses at home.