I agree with mike93lx, but would add that you should check after maybe 10 miles of expressway speed (60-75mph?) driving. The wind and other forces at highway speeds can cause slight shifts and stretching of straps and lines. (If you’ve got a long ways to go before you get to the expressway, then this should be an additional stop.)
As for bow and stern lines, their main purpose is to keep the boats with the car if the rack tie-downs fail. If you over-tighten them you can bend the boat and possibly fold it. Just make sure the knots are tight, and the lines aren’t so loose that they’re flapping around. Bowlines and truckers hitches are all you should need for rope knots.
Long lengths of flat strap that aren’t touching the boat/car/rack can make for loud humming noise. Putting a few twists in the strap will prevent the humming.