headlight cover question

hey everybody,
i was looking at some GT Styling headlight covers or some vinyl overlays for my black stang, i think they'd look pretty good, but im not sure about the legality of these covers. i've seen people driving with them, but some of the places that ive been looking say that they're for show use only. does anyone have these on their car or know if these statements are true or not.. i dont want to buy these and not be able to use them because of some obscure traffic law..thanks:smilie