From a reader:

The things you think of on Monday morning . . . since I was out at a rattlesnake hunt this weekend and have kept some of those obstinate creatures in the past, I can guess that the top-fang tradition comes from human interactions (often fatally) with venomous snakes . . . which I would also bet is part of the vampire tradition as well (the bite does you in either way!) Bottom fangs are more associated with large predators (the big cats primarily, with their essentially equal upper and lower canines.) Now on to matters of statecraft . . .

And another reader:

Fangs pointing down are more civilized, hence vampires. Fangs pointing up are more bestial, like pig tusks, hence orcs, demons and so forth. The less human and less sympathy the reader/viewer are supposed to have for the monster, the more likely the fangs will point up.