Our regular grade is 85 octane here in the Denver area as well. The higher altitude areas offer the lower octane regular because the octane requirement for a given car drops with increases in altitude. I've asked the service guys at my local Honda dealerships and they've told me that 85 is fine for our altitude. However, I've seen information that today's newer cars experience less of an altitude effect.

Personally, I've been using 85 octane in my 2006 Honda Civic Hybrid except for maybe one tank. I've noticed no ill effects.