Most origami-makers will just work with the paper they’re given, most typesetters will use whatever software without much tweaking, most developers won’t get into fine performance tweaking. But in each field, some will do fancier things.

So you think a post about paper grain is more interesting than a post about e.g. spinlocks or lock-free programming?

The article describes the opposite of what you did - grain usually aligns perpendicular to machine direction. After a quick search it looks like this is because of the Fourdrinier process commonly used, where a vibrating (side to side perpendicular to machine feed direction) mesh aligns the paper fibres as paper passes over it.

So this would explain the opposite of what you (and I) have observed with toilet paper.

Toilet paper is built on long rolls, then chopped down, so the perforations are also perpendicular to the feed direction of the roll itself. If this explained it, we would see it tearing more easily parallel to perforations.

Without researching more, my first guess is that the feed direction of the eventual roll is not the same as the machine direction for the paper that goes into it. And with multi-ply paper, do they criss-cross? I would presume so, for strength. You wouldn't want a finger break-through.

Now I'm wondering if there have been any rigorous studies into the orientation that people use with their toilet paper when wiping their arse...