I remember hearing that unless you are using the 92/93 octane within a few weeks that it actually declines in octane. If your fuel is sitting in the tank any longer than this you may consider running the medium grade fuel. I'm not a chemist, this came from a meeting held by one of the local
oil/fuel companies.
With the economy the way it has been, most people I know have stepped down a grade in their daily drivers. I would guess this is pretty common across the board.