A few years ago when I was taking a class with Sibyl Osicka, she told me (much to my surprise) that I was not consistent pulling up the right side of the wool. "The right side of the wool?!" I exclaimed. "What right side?"
She showed me that the "right" side is the one where the strands of the fibers run lengthwise on your strip (top worm pictured here), while the "wrong" side is the one where the fibers run crosswise (bottom worm pictured here).
Why does it matter? Because if I pull the wool inconsistently in my rug, the light will play off the loops differently in the finished rug. So the rug won't look smooth and evenly hooked.
Do you ever pull the "wrong" side? Yes. Sometimes there will be a big difference in color from front to back and you might want the color of the back of the wool instead of the front. Or you might want the texture that the back provides. If you choose to pull the back side, do so consistently.
The long and short of it: know what side of the wool you are pulling and be consistent in any given rug.