There is certainly an effect of propellant temperature on burn rate and chamber pressure. The effect of air temperature at the time of launch is much less significant. If you look at figure 11-7 in Sutton's "Rocket Propulsion Elements" you can see an enormous difference when the burn time vs chamber pressure is charted for a "standard" propellant temperature of 70 F, versus +160 F and -65 F. I would not be attending a hobby rocket launch where the atmospheric temperatures are at either of those extremes. The difference in performance between a propellant temperature of 35 F and 70F would, of course, be much less dramatic, you might not notice the difference.
Even so, when attending a launch when the overnight temperatures are forecast to be below freezing, I usually bring my motors in to the hotel room overnight, and transport them to the field in the cab of my truck. Depending on the size of the motor, you might want to prep the rest of the rocket with the motor in an interior pocket of your clothing until it's time to install. I think an I65 would fit in a shirt pocket, IIRC.
Alan