I was taught, decades ago, in Europe, to saddle a hair forward, cinch snug but not tight, because the saddle would, as soon as the horse moves, settle back into place and so the hairs would be slick and straight.
Then you checked your cinch again and tighten it a little more, didn’t want it to slip back any past that good spot.
That made sense with most horses, that most of the hair where the saddle goes slicks up toward’s the back.
If you settle the saddle in one place and tighten it there enough so it doesn’t move, at times the saddle may want to move up as the horse starts out and may disturb some hairs in places.
That also could be a problem if set too far back and the saddle moves up.
I think that we should go with what we see, how the saddles fit the horse in front of us and how they ride and later what their backs look once you take the saddle off.
I expect rules of thumb how to do things around horses should always come with the warning to be alert, be smart and consider that any horse or situation may need an adjustment to the standard way of doing things.