Backing down a hill is my last option. 4WD, low range and no brakes.
I read an interesting analysis of the back up vs back down debate. Basically, if the center of gravity shifts to a point behind the downhill axle, the tractor may flip. So the heaviest end should be uphill.
Take the case of a tractor with loader and counterweight on the rear. And let us suppose that the counterweight is such that the tractor is front end heavy with bucket loaded and rear end heavy with bucket empty.
In that case, the tractor may flip backwards if driven up a hill with an empty bucket but could flip forwards if driven down a hill with the bucket full.
In a 4wd tractor that is weight balanced front to rear, I favour driving up and backing down. In that scenario I believe the front axle pivot makes it the least stable and would prefer to have the weight transfer to the rear axle, which is fixed.
In the case of a balanced 2wd tractor, there is a good argument that the drive axle should always be uphill (ie: pulling the tractor up) to prevent the unpowered axle from sliding sideways and pivoting around the driving axle.
Just my 2¢. Take it for what it's worth.