I'd caution against going without end lines. Last year I had my Thule roof clamp for the front cross bar slip out on interstate 40. I probably would have lost the boats if I didn't have safety end lines. I now secure the clamps with sheet metal screws into the door frame.

I give the car a good shake by grabbing the racks every time I load up the boats, but still....

I think some rack-to-car attachments are more secure than others. On our '013 VW Golf, there are grommeted holes in the door frame which mate with pins in the Thule rack 'towers' - unfortunately only for the front rack. Still, it seems like a pretty secure system.

I'm also puzzled about the rack spacing recommendations - are there 'stronger points' in the roof which determine the rack spacing? My '97 VW has arrows molded into the roof to indicate the recommended rack positions. On practically all the newer 'aero' roof cars the racks are ridiculously close together, so end ties are mandatory. I wish Subaru still made a plain station wagon like the 80s/90s (??) one a friend had...nice long flat roof!