I asked Cobb this question some time ago. Their response was that you need to run the base map that best matches the gas you use. Period. Our premium gas here is 93, but I asked them if I should run the 91 base map for extra safety, and they said no, run the 93. I asked them what I should do on a road trip if I could only find 91 or 92, and they recommended that I use the 91 oct RT map (no need for base map change), until I could find 93 or higher.