If you're grouping high, move your nock point up. There is considerably more room to move the nock point up before you get into trouble with arrow flight than there is to move it down, assuming you start at a point where bare shaft arrow flight is level.
I think this adjustment is more useful after you have spent a considerable time shooting a bow, and maybe get a new bow or change your arrows, and they're not hitting where you expect them to hit. If you're just starting out, it's better to figure out where your nock point needs to be for optimal arrow flight, and then figure out the trajectory of your arrows. They're going to hit where you're looking at some distance, maybe just not the particular distance you're shooting at the moment.