I know that with the Michelin Pilot SS the rear tire warranty is cut in half if you are running staggered sizes. Not sure if this is true with all brands, but it's something else to think about.
* Mileage Warranty Exceptions
Split fitments - If your vehicle has tires of different sizes on the front versus the rear axles, your tires cannot be rotated as recommended. Therefore, the mileage warranty on each rear tire will cover half the number of miles as the standard mileage warranty for that particular tire line.