I do it every day ! Turn it off and pretend its not there ! They definately like it turned off. Thats valid for cell phones as well. I have been asked on a number of occasions to turn it off since it was just ringing when I got to the booth. As long as the car pc looks somewhat integrated you should be fine. Make sure you are able to show what it is if you get pulled over as well as be prepared to declare it when you step out of the car. I have had my car taken apart on a few occasions (i.e. anything that can be easily removed has been removed, starting with the carpets and ending with the air filter). The customs agents are pretty decent as long as you extend the same courtesy and as long as you dont have anything to hide.