What kind of gas to use on a 2005 Bonneville Supercharge?

A technician at a N. Calif GM test facility explained to me that the computer module on the SSEi can adjust engine timing, automatically, for 87 octane fuel. However, when I know I will be driving in mountanous areas, I prefer to switch to 89 or 91 octane fuel, to reduce the possibility of pinging during long or severe uphill climbs, and, the engine feels a bit more responsive, to me. He suggested using a fuel with cleaning additives every 3-4 tanksful, also, to help keep injection system clean. That is more cost effective than pouring additives in your gas tank.