Never did have a "good" answer to this question about screw slots.

When I did my first rifle aligning the screws drove me crazy! It was a Southern Mtn. Rifle but I wanted the practice.

I also use a couple of sets of screws during a build but when I get to final assy. I tighten the wood screws down, disassemble line the screw hole with wood slivers and wood glue and reinstalled the screws a little shy of indexed.

when everything is set up I can tighten the screw that last little bit! Volia!

Since I can already see the masters cringe from here I won't even mention using bedding compound!

The late Don Brown sold his English Sporting rifle kits with custom screws that were made to be used exactly as Dave stated. Build the rifle, get the slot in the right direction and file it down to size.

On the right guns it is must, on others as stated it is not.

If I am handling a fine English gun, it is something that jumps out to me, either way.

Fleener

My mentor would make his own screws and bolts with extended heads and then cut the slots where they were aligned further in the build. He also liked the fact he did not use modern threads.
For the 18th century type guns I like, I have not seen the evidence that it was a thing. About the closest thing to showing the ideal might be a drawing in a French pattern book showing a clocked head.
I do find it interesting that numerous guns in the Keith Neal collection can be found with clocked lock nails in auction catalogs and every which way but Sunday in other photos.

I make my Phillips head screws go square to the gun, so one line is aligned butt to nose.
I stifled with it but by the fifth or sixth time I removed as just finishing, they slipped out of alignment.