I do pull from the center, but it's possible that on the most recent occasion when I noticed the untwisting to a greater degree, I was not. Someone (either my little girl or the cats) got to some of the yarn, and I may have been casting on with a portion of a skein which I'd rewound after the mishap. It looked good, but who knows which end I was using?!
I confess I do not particularly enjoy casting on. Using my usual method "long tail", I have to do it very slowly or I get an uneven edge. I recently watched the casting on section of Elizabeth Zimmerman's Glossary on DVD, and it was fun to see so many different variations. I also tried casting on in Kitchener stitch by following instructions in "Big Book of Knitting". Not sure I did it right, but it made an incredibly neat but also very tight edge. I'm just not that kind of knitter, I guess! I'm on the loose side. But I might try some of the exotic cast-ons I saw on the E. Zimmerman glossary video. There was a knotted cast on that gave K1P1 ribbing a sort of picot edge.
BTW, I think the wording of the instructions in that book, "Big Book of Knitting" by Katharina Buss, is weird. I can read instructions for something I know how to do without understanding it. I'm not completely happy with that book at all.