I have never worried about it. I have car topped Kayaks for over 40 years now. My first Kayak was a Folbot kit I built in 1974.
As I pointed out in another thread the only thing that I see people do that I don't like is fail to attach the Kayak to a fixed point on the car body. I know with modern cars that it is a pain to find something to tie to but I still think it is a good idea. I use a minimum of six ropes and sometimes 7. Never trust an addon car top rack and check the factory ones regularly. I will for short distances trust the factor racks and not tie to the car body itself but I always assume that they may fail and check them thoroughly before each use.