The reasons behind using 91+ octane versus 87-89 is to keep from causing damage to the engine.
Predetonation is a killer to these engines, when you run low octane fuel you have the potential of predetonation because more octane makes the fuel harder to burn. This is probably ok as long as you don't go over say 1/3 throttle.
Your pcm will also learn the constant knock retardation occurring causing it to use the low octane table. This is an attempt to reduce the timing to stop predetonation. In reality it causes less power and possibly uses more fuel than if you used premium.
00 Regal GS: All kinds of cool stuff.