There isn't variability depending on actual flight path. The mileage between two cities is always the same as far as an airline is concerned. Most use some variation of the haversine formula (straight line distance over a spherical earth between two points of latitude/longitude). You could divert up over Fargo on the way from DEN to MSY and you're going to get the same number of frequent flyer miles (for those programs or elements mileage based) and same service as any other flight.