Also, running in a slightly lower gear is easier on things if the car is loaded down or towing something. Of course the transmission is going to try to be in the tallest gear unless you otherwise override it. Keeping it from going into OD would be better if you are stressing it with a large load.
I also downshift on long downhill grades. There is no point in having to ride the brakes and it can be a matter of life and death in some locations. I don’t think just shifting to shift an auto is a great idea unless you have one of those electronic auto/manual types which is just an automatic with extra controls.