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Aligned with each other is probably more important than vertical or horizontal, but don't try for the extra 1/4 turn or you'll discover just how brittle proper Bakelite is...
Google will give you any number of arguments for both ways (but never diagonal) and also traditional methods of perfect alignment (modifying the countersink depth, or over-length heads and re-cutting the slots) but I'd stick with the 'my home - my choice' school of OCD decision making.
Aren't the screw slot supposed to be vertical?