my rule of thumb (in Southern California, mind you) is no more than 85 degrees. I don't have an under, just because where I live. If I had an under, it would probably be 45. If you don't leave a pet or a baby inside a car....you don't leave your bass either.
This goes for any bass I own, including the upright. If the forecast, or ambient temperature outside is more than 85 degrees, I bring it inside. I get some looks, particularly when it's the upright, but I get way more compliments and "where's your gig" than I do hassles, even with the "crack" security staff (median age of 70 years old).
I don't have this rule for my keys gear, as its mostly all electronics, plastic, and metal.