Here's an Arizona case that illustrates why you should never give police consent to search your vehicle:

The state appellate court has overturned the cocaine-transportation conviction of a Canadian man passing through Flagstaff after ruling the search of his vehicle was illegal.

The reason: The Arizona Department of Public Safety officer who stopped Alvin J. Sweeney, 53, didn't have reasonable suspicion to search his vehicle. [AZDailySun.com]

The suspect refused the search, and although the officer detained him and ultimately searched the car anyway, the whole thing was ultimately thrown out in court. If he'd agreed to the search, the evidence would have been admissible and he'd still be in jail.

Something to keep in mind, even if you've never broken a law in your life. Unless you're the only person who's ever set foot in your car or house, how can you really be sure there's nothing that could get you in trouble?