I leave my dog(s) in my vehicles sometimes. Of course I only do it if I know I will only be a few minutes even if there is a line or they are busy or whatever, like a gas station or the like.
I also have tinted windows so that helps with the heat from the sun.
I also have a/c.
And an awesome alarm. I can have the car running with the a/c with no keys in it. And yes I have an automatic start that only keeps the car running for 10 minutes but my alarm also allows me to double click it, which the car will stay running with the air on for as long as I'd like. Even though I have a/c I still don't leave them in the car when its really hot out, just for any reason it quits on me.

If for any reason I feel even with the precautions I take it is still unsafe then I will not do it.
I figure you take chances at EVERYTHING you do. You never know when anything can happen. Why live your life in a shell? Enjoy it, live it up.