Octane Rating: 85, 87, 91: High Altitude?

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So, tonight I was glancing through the 2001 QX4 owner's manual and saw the following:

"Octane rating tips
In most parts of North America, you should
use unleaded gasoline with an octane rating of
at least 91 AKI (Anti-Knock Index) number.
However, you may use unleaded gasoline with
an octane rating as low as 85 AKI (Anti-Knock
Index) number in these high altitude areas
[over 4,000 ft (1,219 m)] such as: Colorado,
Montana, New Mexico, Utah, Wyoming,
northeastern Nevada, southern Idaho, western
South Dakota, western Nebraska, and that part
of Texas which is directly south of New
Mexico"

Since I live in Colorado (Denver AKA the Mile High City), am I in the clear with 85? Perhaps I'm feeling a little like this is too good to be true. Any reasons why I should stick with the 91 (or even 87)?