Doing the calibration has nothing to do with defining a plane, it's about perspective transformation.

It takes exactly 4 points to get the perspective transformation from one quadrilateral (4 points Wiimote sees) to the other quadrilateral (4 points of screen). Accuracy wouldn't improve just by taking more points. An improvement could be to do the calibration with the 4 "outmost" points of the screen, since you get a better "description" of the quadrilateral.

That's my understanding on the subject, I could be wrong about that since I'm not expert.

http://en.wikipedia.org/wiki/QuadrilateralUwe