For my S60, 91 octane is recommended but not required. The engineers that designed my car know a thing or two about it. For that reason alone, I use 91 octane. However, there was a period of time (about a year) when I did use Regular 87. I noticed the following:
1. Fuel economy was identical -- no change whatsoever. This "old mechanic's tale" that octane affects fuel economy proved to be untrue for my car. I keep meticulous records.
2. There was a tad of pinging at high revs. It might have been my imagination.

Not every station sells 91. When I can't get it I use 89, or if I have time I mix 89 and 93. You don't have to be finicky about this stuff -- just use some common sense. By the way, the guys on CarTalk would probably tell you to go ahead and use 87 and worry about something else.