In my experience, hotter ambient temperatures = better fuel economy. (And I have the fuel logs to demonstrate it - 168K miles w/Big Red and 59K+ miles w/White Pony.) Less engine warm-up time, lower lubricant viscosity, and lower air density (less aerodynamic drag) all contribute to better fuel economy. Summer fuel blend also makes a significant (positive) difference of about 1 MPG in my Mustang.
Fortunately, it rarely gets hot enough in this part of Arizona that I need to use A/C, (except when I have a female passenger
), so A/C's rarely a factor.