I don't think there are any hard and fast rules for catch-lights, other than maybe, "have them."
The lighting affects, literally, every part of the shot. The catch-light is simply the light's effect on the eyes alone. In order to change the catch-light, one would have to change everything else about the lighting used in the portrait - and that's usually less desirable. Phrased another way - you should set your lighting for the portrait as a whole, and then adjust the light position incrementally to make sure that you are capturing a catch-light. You shouldn't start with a good catch-light and then try to light the rest of the scene.
To expand on good lookin' catch-lights: Single catch-lights, as in your question, do look good. But I don't find them any less attractive when clam-shell lighting is used (producing a circle and rectangle, two rectangles, two circles, etc) or Rembrandt lighting is used (producing only a single catch-light on the lit side of the face).
Long story short - beauty is in the eye of the beholder - but I think we can all agree that when it comes to catch-lights, it's only important that they're there.