If you can, always use a higher octane. Most modern vehicle computers work both ways. If you use low octane the timing is decreased when a ping is detected. If you use higher octane the computer senses more efficient fuel is being used and will add timing thus giving you more power and efficiency.
Also after running 87 octane I removed the spark plugs at 20K miles and found deposits. When using only 93 octane and a fresh set of plugs at 20K miles later the ceramic part of the plug was still white and new looking.
The important thing is proper maintenance. If you have a ping or knock and loss of gas mileage, always perform maintenance. Change your air filter, pcv valve, fuel filter, and use a high quality engine oil when changing the oil. If the ping still exists run a fuel system cleaner through the gas tank. Then if it is still there try higher octane. You can also run Seafoam through a vacuum line.
I perform all these thing on a regular basis and my 80K mile V8 gets 24 MPG