I don't allow it to happen. Noses are allowed to sniff as long as it stays inside the car. I'm worried about something hitting the eye and causing blindness or falling out the window, or jumping out when the car has stopped, all options are way to scary!
I expect them to stay in their seat and not bounce around and to be relatively quiet. But I don't really like barking at all.
What do you let your dogs do in the car? What do you expect of them?