Where did you get this "rule"? Which eye should be in focus depends on where you want the viewer to look.
Tetsujin describes factors you may consider while deciding. If you aren't sure, take pictures with one, the other, and both eyes in focus. Pick the image you like best. If you can only get one picture and really can't decide on an eye, try to get both eyes in focus. You might lose some bokeh if you have to stop down a bit. Meanwhile, people often complain about not getting an eye in focus, while no one ever complains that you did get an eye in focus when you shouldn't have.
Over time, after taking many pictures, you will have a better sense of which eye you prefer for different scenarios. It will present itself as an inclination to set the focus point on one eye or the other.