I've always wondered what has more effect more, density or altitude...? If I had to guess I'd say altitude is more of a factor than temperature (because it varies so much more). Altitude can vary by hugely greater percent than the temperature does...
Here in FL in the summer heat when it's in the 90's or 100's we've always ran about 1 second slower than in the winter when it's in the 50's and 60's. We are basically at sea level.
Best times, though, are always on summer blend gas in spring or fall on a cold night.
Breathing in and out is good enough for me.