When I started my apprenticeship to a master carpenter, one of the things that he was adamant about was the "proper" alignment of the heads of the screws that hold the light switch and outlet cover plates. I was instructed 30 years ago, that the screwdriver slot should ALWAYS be aligned vertically because a switch goes up and down and the openings of the outlet go up and down, the screw needs to match. I've always and still align them that way. I've even been in arguments with electricians that (incorrectly, IMHO) finish with them in a HORIZONTAL or even diagonal alignment. Makes me gasp every time I see them "incorrectly aligned."
Maybe I'm being a little "odd" about it, but that is what I was always expected to do when replacing the cover plates. Let me know by commenting below which way YOU feel they should go:
D) None of the above, I don't really CARE