Any decent rack (Thule, Yakima, Malone, etc.) will hold a kayak securely with just straps. I have always considered bow and stern tie-downs as safety additions. Should there be a rack or strap failure they keep your boat from going through the windshield in the car behind you. As such, I always have a bow tie-down, and a stern tie-down also if I am going to be traveling at highway speed.