I have before and I have told them no before. depends on the attitude of the officer, if there nice and just giving me a warning or something I will let them out of courtesty. but if there a dick and don't have anything just trying to find something I will say no. I know the plain sight rule, and if they really think you got something they can hold you and let a drug dog sniff around the outside of the car. Most cops just ask because most people assume they have to.