Back to the OP, I normally fill up with 87 octane in my gasser engines the last tank prior to descending to lower elevations for any significant period of driving time. It really isn't that important with modern engines though as they will detune themselves in order to prevent engine damage. You'll just loose a little power. On Engines with simpler electronics though, such as my Polaris RZR, it will sometimes knock when I push it hard with to low an octane fuel so I'm much more careful with it when I am heading down hill so to speak.
Regarding the side discussion this post has drifted too, it's been a lot of years since I went through school on aircraft power plants, but the simplified basics as best I recall were that detonation at with any given octane rating of fuel occurred at an absolute pressure point, with some variations you had to account for in the temperature area. As a result, the engine having a fixed amount of compression, if the air charge based on air density at sea level brought you to a point just under detonation with a given octane of fuel, the Lower density air at elevation would result in lower absolute pressure values at full compression, and thus your being even farther below the detonation pressure. As a result, you don't need as high an octane rating of fuel to keep the engine from knocking. Of course this only applies to naturally aspirated engines so the eco boost folks need not pay any attention. The turbos on those engines inject a fixed air charge into the engine by adjusting the amount of boost to compensated for altitude air density changes amongst other things.
2015 Ram 3500 Laramie CTD, 4X4, AISIN, B&W Companion Puck Mount
2016 Heartland Bighorn 3270RS, 1kw solar with Trimetric and dual SC2030, 600 watt and 2k inverters.
