If the manufacturer specifies 91 octane, you better use 91 octane.  If you want to use a lower octane gas, you'd probably have to figure out a way to retard the ignition timing so the engine won't ping (detonate).  With the older cars, we just rotated the distributor back so a timing light showed about 2 degrees less in advance.  Newer, electronically controlled vehicles will most likely have to be re-programmed to handle the lower octane fuel.

No harm done, the engine changes the timing.  Doesn't have to be reprogrammed.  BUT you lose horsepower.  87 to 91 octane here in Ontario is +20% ouch.  One of the reasons why I just sold my IS350

Modern cars (you didn't specify your year) will retard the timing automatically. If the engine doesn't knock then you're probably fine, but ultimately you follow what it says in the manual.