Remember that if you are running a higher than stock timing you will more than likely need a higher octane fuel. Also usually during cooler weather you can run a lower octane fuel, but during the summer 91+ octane is needed with the timing bumped up.
BTW... a friend of mine ran his at 18 degrees without pinging in cooler weather, and without the A/C on. He eventually put it down to 14 to be on the safe side though.
OT: Jim are you just copying and pasting that degree symbol from somewhere or is there some trick to typing that in?