Rules governing this type of application are quite vague.
BUT, good rule of thumb: Mount LOW as not to obstruct the driver's view.
Besides, I havent heard of anyone getting pulled over for something like this.
Usually, you'd get pulled over for some other violation and the officer will make mention of it...if he/she cares.
And the addition of GPS and other aftermarket items like HD/Satellite radio systems has become more widespread in the last few years, I believe no-one really notices them any more.