"premium" is a very bad name for the high-octane fuel. It's pure marketing, to be honest.

Unless you're going to a real sketchy gas station or something, all the gasoline sold here in the US is going to have detergents, anti-wear additives, stabilizers, etc. in it. So your 87-octane "basic unleaded" and your 91-octane "premium" are going to be doing similar cleaning of the fuel system.

The higher octane fuel is designed for higher-compression engines (ie engines with turbos, etc) and will prevent predetonation (or "knock") caused by the higher compression. Basically Octane doesn't burn as quickly and the gas won't go boom too soon with the higher-octane fuels. However, Octane doesn't burn as easily as gasoline and therefore high-octane gas actually has "less bang for your buck". It's actually less efficient and you might even get lowered fuel economy if you run 91 octane in a car designed for 87 octane!!!

Bottom line: Look in your owners manual (or on the sticker on the inside of your fuel door if your car has one) and run the octane level recommended by your manufacturer. That'll give you the proper gas for your vehicle.

Now, having said all that, let's talk "flushing the fuel system and cleaning it" (which is unrelated to Premium as I mentioned). Flushing the fuel system should basically never be needed unless there's an actual problem. There is a fuel filter (usually mounted near the gas tank on most cars) that is designed to stop debris and gunk from entering the fuel system of the car. That filter may need replacing (and at 75K I'd recommend it as basic maintenance). Sometimes old fuel will "varnish" or start to thicken & go bad and can plug up fuel injectors etc. If that happens, then a flush and thorough injector cleaner may be needed. For basic maintenance though, it's overkill in my opinion. There ARE fuel injector cleaners you can buy at walmart, autozone, etc. that aren't a bad idea for general maintenance but they don't do a super deep clean (and they're like $5 a bottle, not $300 !!!)

Bottom line: a fuel system flush & deep clean is overkill for basic maintenance.

One counterpoint, however. Some newer vehicles use a new type of fuel injection called GDI (gasoline direct injection). I think the Maxima uses direct injection. This is more efficient than the older port injection, and provides more power with smaller engines. However there are some trade-offs. One of these tradeoffs is a tendency for carbon to build up on the back side of the intake valves and this carbon buildup can cause the valves to stick or the airflow to become turbulent and rob the engine of power. Cleaning these valves CAN be a good thing and improve your engines performance and reliability. That cleaning service can easily be around $300 and may be what your mechanic is referring to. This might be worthwhile for you if you plan on keeping your car for a long time.

The way it works is the fuel injectors will spray the fuel into the airstream that gets pulled into the engine cylinders prior to combustion. With the old port injection, it sprays the fuel in the airstream itself prior to the air entering the cylinder. This fuel/detergent/air mix washes over the intake valves as it moves into the cylinders and keeps them clean. With the new direct injection, the injectors are located with their nozzles INSIDE the cylinders themselves and can more-efficiently spray the fuel at the exact moment it's needed for combustion. This leads to higher horsepower and more fuel economy with smaller engines and is a good thing! But now the fuel/detergent/air mix is no longer spraying over those intake valves and they're not being cleaned like they used to with the older method. burnt fuel, carbon, and other gunk can build up on the valves now since they aren't being continuously cleaned every cycle.

What the mechanic will do then is usually called "walnut blasting" where they will open up the intake manifold and then usee a high-pressure air hose to spray a mixture of air and ground-up walnuts (or sometimes bits of dry ice) at the back of the intake valves. This scrapes off all that carbon & gunk from the valves. It's messy, difficult, and a little dangerous to do (if the valves are completely closed off, that walnut dust can get into the cylinder and wreck it next time you try to run the engine) which is why it costs $300+ to do.