I rarely ever buy fuel until I'm down to 1/4 tank unless on a road trip, then I buy fuel when I have the need to use their rest room. I feel the need to visit the facilities more than my car needs to belly up to the pump.
It wastes fuel to drive to/from a filling station. I fill the tank every time I buy fuel for the same reason, to reduce unnecessary trips to the filling station.
I buy the lowest octane gas that will run ageeably in my car. I did a completely unscientific test in my wifes 1990 Century {back when it was nearly new} and ran a tank of premium, then a tank of midgrade and finally a tank of regular, same long day on an 835 mile road trip. The results were less than 3/10 mpg from best to worst FE, and I forget which was best. At the different prices for fuel, regular won the coin toss. It ain't
just the FE numbers to me, but the out of pocket expense.