Patrice is absolutely right. Because of the "rules" we are often too intimidated to try something new for fear of the Earth spinning backward. I was so scared of FMQ before I tried it that I would get this horrible knot in my stomach even at the thought of finishing a quilt top... How will I quilt it? What if I mess up my quilt by attempting a new technique? and about a million other questions including will the walls cave in and the QP take me away to place me on display for public ridicule? Finally I tried it, and when I messed up, I was able to fix my quilt pretty easily with the aid of my handy friend Mr. Seam Ripper, and when I crossed over my lines of stipple the walls didn't cave in and out of all of the people who have seen this quilt, not one has ever ridiculed my work. Instead, I was asked for one many times over, and 2 even asked me to teach them how to do it. :shock:
No one should ever be afraid to try something new just because of the rules and the QP... neither exists in your private sewing area until you are ready to allow them into your quilting processes. As a matter of fact, the only time you will ever encounter them is if you enter a quilt into one of their quilt shows, and then it is different because you have invited their critique.
As far as why we don't cross over the previously sewn lines, I have heard it can cut those threads, but the answer that made most sense to me was crossing over the lines breaks up the background pattern you are trying to accomplish, thereby preventing the flowing movement of the eyes across the quilt... sort of like hitting a visual speed-breaker.
Maybe someone was just trying to make theirself feel more important by making it more difficult on beginners to the technique?? Machine stippling hasn't been around very long has it?