If the engine runs on 89 without pinging, (it probably will unless there is a lot of carbon buildup) there is nothing to gain by burning anything higher. If there were still vehicles on the road that used 68 octane, 91 might cause exess carbon formation and maybe even shorten exhaust valve life in them. With modern engines and "gasoline", the spread is less than 4%. Not wide enough to cause a lot of trouble as long as you use a high enough grade.
