I'm no 'expert' but I don't think circular motions have a huge effect on swirl marks. I'm sure 'swirl marks' can happen because of it (cob-web marks), but if there were small enough dirt/dust particles on the paint, you'd get scratches if you went back and forth anyway. But it'* impossible to avoid swirl marks. You can prevent a lot, but you'll always get them. As most are micropscopic enough that you can't see them without strong light. As long as you use the right products, from the wash mitts, microfiber cloths, polishes, etc. and use care it shouldn't matter if you use circular motions vs. back and forth. And they do make good products to remove swirl marks.
I use back and forth for the claybar since I find it'* easier, and the directions say to. I have always mostly used circular motions when applying and removing waxes.