it depends. depending on kayak, racks, and what you're doing you don't need anything on the front and back.
a kayaker/outdoor friend just showed me how to use two ratchet straps and no front or back lines. i was nervous at first, but once he showed me it's so freaking easy I can't believe it was so cumbersome before. but I can only fit one that way, so it'll be kayak racks in the future to tilt them for more space.
I use two tie down strap loops that are bolted under the hood and a similar type that you close the rear door on for the back.
The front straps fold under the hood when not in use. They all work great and are probably the easiest to use, instead of having to crawl under the front/back to attach somewhere and risk the rope causing paint damage.