somewhere in one of my books - maybe kindig or shumway, or maybe someone else entirely, I forget - I read of some really extensive tests done in the latter half of the 20th century loading sprue up, down, and sideways and no discernable difference was noted.
that said, even though i know it probably makes far less difference than rifle operator error, i load sprue up just so i can see it, but i admit it's pretty often cocked a few degrees to the side
i cast my own and did a bunch of my own testing just comparing up vs down and could find no difference
i also used to use a rock tumbler with no medium to tumble my balls until the sprue disappeared and that remained was a small raised spot with a little dimple. IMPORTANT: I actually found these to be about 0.001 inch larger than untumbled balls and--believe it or not--I could feel the difference when loading - they just loaded tighter. because the tumbled balls were no more accurate or precise than untumbled, and because it took another 4-8 hours to tumble them smooth, and because they loaded tighter, I stopped tumbling and I still kill plenty of deer.
at recommendations of others here, i was tumbling to identify voids in my cast balls. i never found one because you as the caster just 'know' when a cast isn't right - too little lead, too slow pour, or whatever, and that ball goes back in the pot before ever getting a chance to cool.
go with whatever works for you!