It might seem natural for parents to expect that once their teens are licensed it's the end of the process, but that's not a safe assumption to make. It actually tends to take much longer (often six months to more than a year) for teens to hone their defensive driving skills and develop the safe habits it takes to successfully navigate all the curveballs that might come their way on the road. Then there are also incidents of peer pressure to contend with -- whether that's encouraging a fellow teen to race or mocking his or her careful habits. Lessons concerning safe driving habits should definitely be reinforced continually by parental oversight and participation.
Maybe they've started rolling through stop signs, begun skipping their seatbelts or stopped checking their blind spots -- remind them that such behavior is unacceptable. Crafting a parent-teen driving contract is a good idea, too. It can include best practices you want them to follow, promises you want them to make and penalties for any violations they incur.