After a great day on the river, imagine driving home and hearing a loud pop, then looking in your rear view mirror as your canoe or kayak bounces off the car behind you. Tying down your canoe or kayak is your responsibility and needs to be done right.
Either straps or ropes are fine to secure your boat to the roof of your vehicle. Always use an additional bow (front) and stern (rear) tie down in case one of the other ties or your racks fail. The following video shows an EASY way to improve the way you tie a canoe or kayak to a car. Nylon webbing is really cheap and available at REI, Home Depot, or most outfitters. If you have straps that are too long, you can cut a few inches from the end.
When you are done tying your boat down, you should be able to rock your car by pushing the end of your boat. If your boat still wiggles, you need to secure it better.