Question: Must a bar mitzvah boy buy a mirror to ensure that his tefillin are perfectly center on his head?
Answer: The holy sage, HaGaon HaTzaddik Rav Chaim Sanzer, the author of the Divrei Chayyim, ruled (Divrei Chayim, cheilek II, Orach Chayim, siman 6) that such a custom is foolish and uncouth (divrei burot).
He argues that even if the shel rosh is not placed exactly in the middle of one’s head (at the hairline), one has still fulfilled the mitzvah. This argument is based on the halachic tradition that there is room to place two sets of tefillin on one’s head and on the fact that there is no maximum measurement for the width of the tefillin shel rosh. (See Tosafot, Eruvin 95b and Bet Yosef, Orach Chayim 27.)
Accordingly, a person need not be worried about positioning his tefillin perfectly center on his head. Carrying a mirror around to do precisely this is therefore unnecessary.