Some mixtures can be blended more quickly, easily, and uniformly if the cook stirs in only one direction. However, save for the negligible influences of human physiology and geodynamics, it doesn't make a hoot of difference whether you stir clockwise or counter-clockwise (unless you believe in that old superstition that stirring counterclockwise brings bad luck).
Human physiology? Because of the anatomical design of the arm and hand (as well as force of habit), the vast majority of right-handers can stir more adroitly in a clockwise direction. The opposite is true for left-handers. Because right-handed chefs and recipe writers outnumber their southpaw colleagues by about nine to one, one can understand why the "stir clockwise" commandment became engraved in many cookbooks.
Believe it or not, geodynamic factor called the Coriolis force affects the ease of stirring - but again, to a minuscule degree. The rotation of the earth as it pirouettes through space creates an interesting phenomenon: Vortexes such as kitchen sink whirlpools and tropical storms generally spin counterclockwise north of the Equator and clockwise on the lower half of the planet (unless the body of liquid or gas has strong counter-currents that could give the incipient vortex sufficient momentum to start swirling the wrong way). Ergo, a counterclockwise circular force - albeit minute - is exerted on the fluid food in a bowl in a Northern Hemisphere city like Cleveland or Copenhagen. This bonus power imperceptibly increases the velocity of the whirling mass if the Northern Hemispherean is stirring counterclockwise. Conversely, one who follows the "stir clockwise" superstition has to expend more energy - perhaps the equivalent of one calorie per century.
** Food and Culture