Gotta remember, the OP is already a little hesitant. So let's put this in perspective. Line a car up with a 91 tune against the exact same car with a 93 tune. You will never see a noticeable difference (if any at all)....GUARANTEED. Now, isnt it better for HIM to tune on 91, then run 93 for his own peace of mind? He isnt giving up anything at all performance wise, but gaining safety and peace of mind. He also doesnt have to worry about getting crappy gas. Let's be honest.......what's the best decision for HIM.......?
If he's concerned about octane, he can monitor his OAR a few miles after a fill up. I do that sometimes if I'm not sure I added enough ethanol. There was only one time my OAR's went above -1.00 and it's because I tried to cut 87 with E85. I had to go back and add more ethanol to get -1.00.