Throw spark advance at it and you complicate the process but the logic is the same. There's a little advance left in the factory curve to account for all the conceivable locations and altitudes, but GM has about maxed out the curve for low test gas. They have to maximize the gas mileage. So if you're bone stock, and unless the manufacturer determined high test is required, you're likely wasting money and power going to higher octane. Unless you know it's knocking and the ECM is retarding the timing to compensate, fight the temptation to give her a treat. That said, just because the pump is marked with an octane number, doesn't mean they are all the same. We've all had bad gas that knocks. Up north it's more common when they switch the blend seasonally. Racing gas is more consistent that way and makes the small amount used worth the expense.

The one time to re-consider your fuel choice is putting it away for the winter. Up here, only the highest octane is without any ethanol (marked right on the pump). "Carb defender" is on the market to help reduce the effects but articles in http://www.powerperformancenews.com/ suggest fresh gas with ethanol (ie driving it) is not a problem. Leaving it sit where the ethanol attracts the water and causes corrosion is. Suggestion is to fill it to the neck to minimize the available area or drain the tank.

I'm not a fan of either since gas breaks down over time and I don't really want a tank full stale gas in the spring. Draining it entirely leaves things entirely exposed too. So, my plan is to add fresh gas without ethanol when it gets warmed up regularly over the winter, trying to freshly pickle it each time.