OK, I am not one of the two “E. F. Hutton’s of leatherworking” on this forum (Chuck Burrows and James Rogers are), but here is what I’ve found from my experience.
When you bend leather over a 2 x4 or when you bend it back and forth while dry to soften it, you are breaking down some of the leather fibers. That can be a good or bad thing DEPENDING on what you want the leather to do after the project is sewn. Generally speaking, if you WANT the leather to retain its body after you have sewn it up, then you don’t want to do that. ALSO, you risk breaking/cracking the smooth surface of heavier leathers when you dry bend them. There is NO way to repair that and though the leather will still “work” for most things, it just looks terrible when that happens.
Allow me to give you an example. When sewing sword, knife or bayonet scabbards, INSTEAD of bending and breaking down the fibers, it is better to dampen the leather with water (and with distilled water if your local water is bad) so it will “bend enough” to sew it. That way the leather will be firmer and retain its eventual shape of the scabbard when it dries. You can also wet form leather for sewing/making other things like leather dice cups, belt/strap loops, cartridge boxes, etc.
Now there are some parts of over the shoulder straps or belts that I do bend back and forth on themselves, but it is always after I have applied some kind of oil or leather dressing. I like to do this on the upside down “U” part of the shoulder strap that goes over the shoulder. That way it won’t rub or chafe you as much. I will also do it on the areas of leather belts that fit the sides of the body, but NOT so much where the strap goes through the buckle. I want that part to retain much of its body so it doesn’t curl up like a pig tail. I also do it on the areas of a leather sling that are bent back on themselves.
Bottom line is the safest way to soften leather to sew it is by dampening it with good water. Then you can break down the leather fibers in areas that you need or want them later after applying oil or leather dressing.
Gus