That said, the actual federal test procedure calls for a "prep cycle" where the ECM gets a chance to learn any necessary fuel trims over the same drive cycle. Ford has recently started skewing the stoich point in their calibrations to something closer to E10. In theory, it doesn't really screw up the EPA tests as long as a good fuel trim learn happens on the prep cycle. What this does is help make real world fueling a bit more precise as the customer fills up at some random gas station with an unknown blend.
I prefer to calibrate for the fuel that's actually in the tank when I'm looking at the car. I will often take a fuel sample in order to determine the alcohol percentage and stoich point. If I don't have an exact measurement, I still tend to SWAG it based on what I know about the available fuel in the area.