Generally everything said is true. From a practical standpoint however, it's easiest to run what they sell at the pump.
Is this going to hurt the engine? No, not unless it detonates (pings). Lower octane fuels are more volatile, hence they ignite easier hence detonation occurs at a lower compression, and lower heat level. In normal driving, this is the criteria for using gasoline in your car. Spending any more money doesn't buy any more horsepower. In fact, the reverse is true. Lower octane fuels potentially offer more horsepower since they have a higher thermal output. Engines are heat machines...they convert heat energy to mechanical energy. The hotter they get, the more horsepower they produce...problem is, this same heat is destructive to the engine's component parts.
Now if the engine is subject to routine hard acceleration, climbing steep hills, or alternately the cooling system is substandard (overbore engines using factory cooling system), then there is a reason for using higher octane. The hard driving will result in detonation unheard with potentially damaging results. In this case, an ounce of protection is worth a pound of cure. Thus, in situation buy additives and the highest octane you can.
When you get arount to rebuilding the heads, get hardened valve seats as this is a weak area that can be easily improved during the rebuild process. There is normally no reason to jump up and tear your engine down just to accomplish this...unless you drive hard and fast.