There is a counterintuitive thing that at altitude you can actually run lower octane. Something to do with the density of the air IIRC. I remember being in Colorado or Wyoming years ago and they had 85 and I asked the gas station guy about it. Seems totally backwards but I'm no physicist
Vehicles with high compression are more apt to have pre-detonation. This is especially the case in forced induction performance vehicles. At altitude where you are getting even LESS air and LOWER overall compression, I'm not saying you should run a lower octane, but I certainly wouldn't go higher.
With altitude comes hill/mountain climes, which the added load is more likely to cause pre-detonation. To my previous point though, this would be more so at the bottom of the mountain where you still have good air. By the time you get to the top, the high load and thin air would likely cancel each other out.