I do sometimes leave the dogs in the car. Thing is, I don't have much choice in the matter! The town I go to for basic stuff like grocery shopping is 2 hrs drive (one way). Often we go in overnight to do all our errands. So clearly the dogs have to come with.
That being said, I'm pretty paranoid about it. I always park in the shade, roll the windows down as much as I can, and RUSH through my errands. If my OH is with me, often I will stay with the dogs so I can leave the doors open, or else I will go check on them halfway through the grocery shop. I make sure it never gets too hot for them, that's for sure!
Otherwise, we pretty much spend all summer at the beach, and playing in the pond. I think my dogs will survive.