That's a good question.
When using roof racks lashing the yak to the racks is good enough for some people. If I'm travelling a long distance or plan to exceed 40 MPH I like to use a bow & stern line as well.
The most important thing about using a bow & stern line to remember is to avoid a "negative" angle. By that I mean your bow or stern is mounted onj your roof in a manner that the bow or stern line is angled in to the where it meets the car vs. out.
I hope that makes sense. I will try to find a link to an on line articvel that explains it better.