I think the way that the fuel recommendation is worded in the owner's manual can be a bit confusing... EDIT: ^I see GR8 BUY beat me to the quote.Here's a decent explanation of octane ratings, from Wikipedia.
I've been using 87. I tried a tank of 92 but, didn't notice any changes (mpg/knocking) so, I figured 87 would be just fine. My previous car was a Subaru WRX. This car required premium fuel, and demonstrated performance differences, with a slight difference in mpg, when I used lower octane. That said, I put 87 in the car anyway.
2008 Altima Hybrid | Sonoma Sunset | Charcoal Leather | Tech Package | Full Nose Mask | Moonroof Wind Deflector | Madico Charcool Tint - 35% Front, 25% Rear | MOMO Stealth Airmetal pedals | 16" O.Z. Caravaggio Rims | Stillen Hood Dampers