If you use a hammock ridgeline connected between the two ends of the hammock. You can adjust the amount of sag by adding more tension or by loosening the ridgeline, so each time you use your hammock it is exactly the same as you had it the last time you used it.
If you imagine everytime you set up your hammock the tree's you tie to are different widths apart then each time you use your hammock the sag will be different to. But with a ridgeline will always be the same.
If you have the correct sag with your hammock and sleep diagonally in them you sleep flat. Mine I can even lay face down if I want to.