The first thing to cover is a "knock sensor" -- this detects "engine knock", which can damage the engine, and to prevent this, if it is occurring, the sensor is connected to circuitry to electronically retard spark timing (and can be successful as long as the fuel is at least 87 A.K.I. -- see below). Advanced timing improves power and fuel economy, but makes engine knock more likely under certain conditions (hot weather and heavy load, such as accelerating); retarded timing prevents knock, but reduces power and fuel economy.

Next, Volvo's engines are designed (re compression ratios, etc.) so that they normally won't knock under the worst conditions (temperature and load) if the fuel is at least 91 A.K.I. (the new term for what most folks still call "octane" -- it stands for "anti knock index") and is exactly the number posted on gas pumps through the U.S. (= (RON+MON)/2). This is why 91 A.K.I. is "recommended", per your owners manual -- its use means that the engine won't ever (normally) have to retard timing, and you'll get the most power and fuel economy possible.

However, the range of capability of the antiknock/spark retard safety system is limited -- and so 87 A.K.I. is the "minimum" you need. You should always use at least 87 A.K.I. (per your owners manual), so that the system can protect your engine. If you use less than 87 A.K.I., knock will be more serious and beyond the capabilities of the system to be prevented. However, use of such fuel (i.e., less than 91, but at least 87 A.K.I.) means that your engine's spark timing will be retarded from time to time, especially when you really need it such as accelerating or driving up hills, and you'll get less power and economy from your car as a result.

Of course, how often and how much retardation might occur (if you use less than 91 A.K.I. fuel), and how much fuel economy you might lose, depends on a lot of things.
1) do you drive hard, or easy -- hard driving demands more power; if you drive easy, you may never encounter engine knock, and spark retardation.
2) in the same vein, what's the terrain? If you only drive on flatlands, without any long highway grades uphill, again you may not need the extra antiknock benefits of =>91 A.K.I.; but if you drive in mountainous regions, I wouldn't touch anything less than 91 A.K.I.
And so on -- your driving isn't the same as the next person -- it's a personal choice.

So you have to decide whether saving the ten cents or so per gallon is worth the loss of power and fuel economy. When you fill up, how much do you fill? Twelve to fifteen gallons, maybe? That's a buck fifty? Given that you're paying sixty dollars, is it worth it?

And remember, the more the price goes up, that ten cents per gallon becomes a smaller fraction of the total amount you pay for gas.

I use 93 A.K.I., because in my part of the country, we can't buy 91 A.K.I. gas -- our choices are 93 or 89 (or 87, too). You're lucky if you live where they sell 91. I wish I could buy 91, because 93 is a little wasteful, but I'm not going to go below 91, because I drive hard and figure that my car needs the >=91 rating.

So now you know the facts -- you decide.
Hope this helps.