doesnt really matter too much. normally i put it at the top (if using the first slot right under the cd/dvd cage or the bottom (if using the slot at the very bottom of the case to maximize airflow into the case while still giving a bit of airflow around the hdd. maximum drive cooling would be centered however its not really needed.
in short... just about anything will work. centered = better hddcooling but messes with the case airflow a bit, off center = less hdd cooling and better case airflow. either of them is acceptable.