Yeah. Everbody knows you're supposed to wait until they bite.
About three years ago, I was pulled over by an OHP trooper for exceeding the speed limit :/ while driving on I-35 through Oklahoma City. It was after dark and I had my wife, daughters and two dogs with me. The dogs growled and the trooper was visibly shaken and he put his hand on his side arm. He made me get out of my van and sit with him in his cruiser while he wrote me up. He was pretty surly with me the whole time. I'm sure his intitially evident fear and subsequent anger was because of the dogs.
Regardless of my thoughts regarding the trooper's demeanour, I travel with the dogs in restraint harnesses secured to seat belts, both then and now. It's much safer for me because the dogs can't roam around the vehicle and cause problems. It's much safer for the dogs because they can't be thrown around in the event of an accident. And in the light of both my experience in Oklahoma City and the experience of the Smoaks in Cookeville, TN, restraining your dogs in the car has definite advantages if you are pulled over by the police.