Offhand I’d advise – no. It’s to your advantage to keep the boat as light as possible since wind and the motion of the car can result in the damaging forces that Vincent mentioned. Anything in the kayak that’s free to move will move in the opposite direction of motion so if you have to brake in an emergency, that lose, sharp paddle in the cockpit will move toward the bulkhead at a high rate of speed. The most precarious days of your boat are the days it’s on top of the car. If you’re travelling at 80 kms/hr on a two lane road and you meet a semi on the other side moving at 90 kms/hr, your boat experiences winds of about 170 kms/hr. Even if it’s only for a short time it’s still a significant force. Better to have the boat as light as possible and tied in four places. Bow and Stern lines and a good strap at each roof rack. As Nick mentioned, put a good cockpit cover on while it’s up on the car. It makes the boat more aerodynamic, saving you some gas and it keeps any water out which can become a wave sloshing from bulkhead to bulkhead as we brake and accelerate.