Centered is nice, but not important (unless the tip passes really close to the edge of the stem).
1. With the valve closed, the roller should be inside the center (close to the valley)
2. At 25% lift the roller is centered on the tip
3. At 50% lift the roller reaches its farthest point away from the valley, and stops
4, Reversing, 75% lift when centered again
5. Full lift returning to position 1