Hello @stir_choc and welcome to Seasoned Advice! You really gave us a tough question! I have to admit that I had never heard of this. However after much digging it appears that there is in fact a very scientific explanation for the reason behind this.
When I first started researching this, I came up with many results for recipes that gave the instruction to stir in one direction only. Some stated that it was important or that the dish would be ruined if not done correctly, but none said why.
Thanks to a similar question on the SE Physics site, I was able to dig up more information. As @Jefromi and @Aaronut noted, temperature is the most important factor. But there are other factors involved, not only for chocolate but for other mixtures.
Simply put, by stirring gently in one direction you are creating a laminar flow as opposed to a turbulent flow. This helps to keep from incorporating air bubbles and lowering the temperature. Also, stirring in one direction allows the protein molecules to form into strings. If you stir in one direction and then reverse direction, they will form into balls.
The flow and shear are important in that they affect aspects such as texture, viscosity, mouthfeel, etc. With chocolate, probably more so than other foods, you would want the flow to be as laminar as possible.
This link has a ton of information on physics in food production and is primarily focused on chocolate. And fortunately it is written pretty much in lay terms. (There is a wealth of information and studies available but most are published by physicists and way out of my league.)
This link explains a little about how stirring in one direction only affects the protein molecules when making dumplings. (And what that means to the finished product.)
So, while I was skeptical about the whole idea, it appears that there could be merit in it. That said, I think it would be much more critical in commercial production than in a home kitchen. As I read on many posts, most people get good results without incorporating this. I may just have to give it a try and see if I can distinguish any difference! :)
Hope this helps! :)