You can "think" whatever you want, but if you watch the needle on a regular machine or long arm, the thread makes a different size loop when making the stitch, this is what causes the thread to shred or break or not make a "good" stitch.
As I said before, you can go in all directions, but the emphasis was to go left to right, I think this shows up more on the pantograph designs than on free motion quilting. This was told to us by not only the person teaching the long arm techniques, but the engineers as well. I took a half day session with the engineer that designed the machine.
I often stipple in an all over direction in small areas at a time, I don't see much difference in the stitch, but if I am doing a stipple going from right to left, I can see some difference in the stitch formation. Possibly I see it because I was trained to see it as I do work on the machines, and still do all the maintenance on mine. Since I look for other things, I am probably looking more closely, and am more critical of the stitch formation than others would be.
However you stitch, the long arms and mid arms are a great boon to quilters every where. I would never tackle a queen or king size on my regular machine, too much strain on my neck, arm and back muscles.