Well, your manual says use 91 RO, but the reality is, use what works best for you through "old fashioned" trial and error and consistantly from the same place. Found that my Sporty will run "Medium" 89 RO no problem...even two up. Can't say the same for my Katana, which would run 89 RO but if two up, heavy throttle or hot days only 91 RO will run right.
If your bike is sensitive to low octane gas it will ping readily under heavy throttle, hot days or riding two up. If that is the case stick with highest octane, no point in pushing a bad position. Also, if in different areas you normally don't travel, do "not" take a chance and go with 91 or highest available and along well traveled (popular) stations, with no tanker unloading while you are there to minimize chance picking up contaminated gas...JMHO.