At first, it seemed to me to make sense to carry one's powderhorns on the side opposite one's dominant / shooting hand. (I.e., horns on the left, if you're a right-handed shooter.) The reason being that the sparks, etc. from one's lock theoretically wouldn't be falling down in the vicinity of one's powderhorns that way.
Then, one of the old-time regulars here clued us in to the fact that when a right-handed shooter is shooting a longarm, he's got the butt of the gun against his right shoulder, and is bracing the gun with his left hand. Thus, the lock of the gun is actually a good ways LEFT of the shooter's right shoulder. Given how one actually holds the gun while shooting, any sparks from a right-handed shooter's lock would probably fall closer to a horn on the shooter's LEFT side than on his right.
Just some ruminations, these.