there's too many variables to give a battery temp average. EX, Charging, streaming google music, running ingress (uses GPS, CPU, GPU, and Data), and phone mounted on a sunny dashboard I've seen 114°F. Sitting on a table doing nothing the phone is ice cold. Charging with the screen off the phone is just warm.
If you really want to compare, you have to eliminate these variables and tell people exactly what you were doing with the phone so that they can do the same with theirs. If you don't do that your comparisons will be apples and oranges. In general, 91°F is nothing to worry about.