Keeping your instrument safe when you are not practicing is very important. Wooden instruments cannot handle temperatures that are too hot or too cold. They will crack and warp if left in extreme temperatures.
In a vehicle or outside your home --my personal rule of thumb is: if you would not want to stay in a car for 30 minutes without the heater or air-conditioning on- don't leave your instrument! I have vivid memories from my childhood-- after concerts my family would go to DQ to get a 'treat.' There I was, with my violin strapped over my shoulder, enjoying a Blizzard.
When at home --keep the instrument out the path of direct sunlight from a window and far way from a heater/ air vent. Those temperature changes really do make an instrument go out of tune, and worse... crack! Taking good care of your instrument gives it the chance to live for hundreds of years.