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.