A lot of my cars specify 91 octane or better, but I always run 87 octane, works just fine. If you can make use of better octane gas by running more boost and/or timing, then it's worth it.
100 octane gas will make an old carbureted Chevy run better because you can advance the ignition timing without pinging. I wouldn't waste the gas though in something like that unless I was running domed 12:1 pistons or higher. Or if it had a set of T88's on it.
