Where I live the average altitude is around 400 ft. above sea level. Gasoline at 91 octane is not available. The octane ratings skip from 89 to 93. I use 93. The higher the altitude, the less octane needed to avoid knocking, if everything else is held constant. In the higher elevations (e.g., high plains), 93 octane is not available. Above 8000 or 9000 ft. you might not even see 91 octane.

 

If you shoot for 9.2:1 corrected and a CCP of around 190, you can easily build a reliable street motor that runs well on pump gas. There's really no reason to go beyond that for a street motor. If you're starting with 10.5:1 static, you've got lots of great cams to choose from that will give you a wide, useable power band.

You can go higher, but combustion chamber design and the tune become more and more critical.