Have you noticed how rolled paper towels never rip straight down, like you want them to? Ever wondered why?
Correct! Because of the way fibers are aligned in the paper. You can imagine that checking for the fibers in the paper or even cardboard for book covers is extremely important. If done correctly, the fibers will help the book stay flat and will fold easily. If done incorrectly, the fibers in the book may bow or warp it.
So, the first thing you need to do is to check your paper / cardboard using a tear test. Just tear a corner and see of the paper aims sideways or follows the direction of you hand easily.
Second option is the bend test. Just arch the paper sheet between your palms and try to feel the arch radius during the max resistance. Then turn you paper 90 degrees and repeat. Define which orientation showed showed less resistance and use it for your crease.
And finally the fold test works best if you have extra paper or if you work with thicker papers. Simply align the corners and fold 2 pieces of paper in 2 different orientations. Then compare the quality of the crease. The fold with aligned fibers will look sharper and cleaner.
And remember! Always align the your bend or crease parallel to the fiber in the paper.