Provided I stay away from Sams Club and reservation stations with her X5 I think it's pretty consistent. My PW runs fine on 85 octane at 5-6K feet, but you usually on find that on the reservations. It's usually 86 or 87 elsewhere.

Altitude definitely does play a factor in dropping down to a lower octane rating. From reading (a few years ago), oftentimes, a 2,000 increase in elevation was enough to drop an octane rating with no appreciable loss in power or premature detonation due to the lower cylinder pressure.