My entry point for running 93 in the summer is when fuel prices are above $3, with the extra dime hit to move up a grade. That's when the efficiency started paying for itself with premium, based on long time ago spreadsheet I parted with. Otherwise the 91 tune works just fine for me in the summer months.

In the winter months, I take a pretty big 15% decrease in fuel mileage with a longer time to reach operating temperatures. Daily temperatures in the teens or 20s and 87 Octane is the right call for me. Just too much warm up time and I'm sure running in open loop a ton with fatter A/F mixtures. I'm also not as inclined to want to mash the throttle with snow covered, icy and wet roads.