I would run the lowest octane the owners manual recommends. Using a higher octane may decrease your MPG by changing the long term fuel maps stored in the KAM (keep alive memory). Fuel maps are stored in your PCM so at different engine loads and RPM's they don't have to be constantly adjusting the air/fuel ratio. A change in the octane or adding ethanol to the gas will cause the fuel maps to be relearned in KAM. During this time (about 3 tanks of gas), mileage will suffer. In the end, a higher octane may not hurt your mileage, but it will most likely not improve MPG.
Some manufactures design engines for high octane fuels but state you can run lower octane fuels also. The Toyota Highlander Hybrid V6 manual for example states this. People have reported better MPG with the higher octane in that vehicle.
I added 93 octane to my Ford Escape Hybrid by mistake and saw a quick drop in mileage right away. It wasn't as bad as ethanol, but I noticed the drop in MPG on my Scangauge throughout the tank. If you want to improve your mileage, you can monitor your driving habits with a scangauge, see scangauge.com about this device if you like. If you really want to learn how to improve your mileage, visit CleanMPG.com and post to that group. CNN has been doing stories about that site and Hypermiling. There is an article on that site I wrote called "Hypermiling The Ford Escape Hybrid" but the site is for non hybrids also. | 
