:agree That's the way I've always heard it, that octane boosters are rated such that 1 (octane boost point) == 0.1 gas rating point.
Also, I don't know much about the mechanics and such, but similar to what some others above have said, I've always heard that using too low an octane gas can cause damage in older engines; in newer engines the car will usually adjust but you will generally get worse fuel mileage, so you are not saving much if any money in the end. And of course, too low and it pings and stuff. On the other hand, I've heard that using a higher octane gas (note that this is assuming the car is stock, not talking modded) doesn't do anything for the car, with the possible exception of if the higher octane gas has more detergents in it. So if the family cruiser is rated for regular, you aren't going to get better performance or anything by running premium.
Anyway, after hearing stuff like the above constantly, I just stick with what the manufacturer recommends for a car. I figure this is safest. I always fill my GTO with premium, same as I did for my Z28 before. Though if I was out on a trip, was in the boonies, and the only gas station around did not sell premium, at least I could fill up on midgrade to get me by.