Some of my analogies get used rarely, others often, but the one all of my string players without exception hear over and over is this: treat your instrument as you would a pet.
Despite this warning, every year people forget and arrive in my studio with instruments that have undergone incredible stress due to temperature changes from being left in cars on hot or cold days. Usually the damage is limited to out of tune strings or stuck pegs, but the risk of cracks and open seams is ever looming (not to mention the possibility of theft).
Each stringed instrument is unique. Don't be responsible for destroying the beautiful creation that you are lucky to call yours. Take it with you on your errands, to lunch, or anywhere else you are going on your way to and from lessons, rehearsals, concerts, and gigs if you are unable to swing by home.
Bottom line: if you wouldn't leave a living creature in the car, don't leave your instrument.