Question: I got a leaflet through my door advertising tefillin and mezuza checks, saying that one must check them during Elul. Do I need to check them every year?
Answer: Rambam (Tefillin, Mezuza and Sefer Torah 2:11) writes that providing one’s tefillin come from a reputable sofer one can safely assume that they are kosher and they do not need to be checked even many years later. Similarly, the Tur (OC 39) and the Shulchan Aruch (OC 39:10) write that a good pair of tefillin does not need checking providing that they are worn regularly. Otherwise, they should be checked twice in seven years as we are concerned that they may have gotten mouldy (Magen Avraham OC 39:15; Aruch Hashulchan OC 39:6).
The Magen Avraham (39:14) and Mishna Berura (39:26) write that as sweat can permeate the tefillin and ruin them, they should be checked periodically. Similarly, the Aruch Hashulchan (ibid.) writes that they should be checked regularly as the ink in his day would crack easily (See Chayei Adam 14:20; Mor Uketzia 39).
However, R’ Shlomo Zalman Auerbach (Halichos Shlomo, Tefilla 4:36; n52) writes that as tefillin nowadays are generally manufactured from thicker hides and better quality ink and parchment, one should not have them checked unless one has a specific reason to.
The Gemara (Yoma 11a) writes that while public mezuzos only need to be checked once every fifty years, mezuzos on private dwellings should be checked twice every seven years. Rashi explains that we need to check to ensure that the mezuzos haven’t been spoiled or stolen. Thus, Rambam (Tefillin, Mezuza and Sefer Torah 5:9) and Shulchan Aruch (YD 291:1) write that mezuzos should be checked twice every seven years. The Mateh Ephraim (581:10) and Kitzur Shulchan Aruch (128:3) write that it is commendable to check one’s tefillin and mezuzos every year during Elul. R’ Ovadia Yosef (Yechave Daas 1:49) adds that this is particularly important as there are many inept sofrim who make mistakes. The Aruch Hashulchan (YD 291:1) writes that this specifically applies when the mezuza is prone to dampness.
Nonetheless, R’ Shlomo Zalman Auerbach (Halichos Shlomo, ibid.) writes that mezuzos wrapped in proper cases would not need checking this often.
In conclusion, it is most important that one buys good quality tefillin and mezuzos from reputable sofrim which do not require frequent checking. Good tefillin that are worn regularly do not need to be opened and checked unless one suspects that there may be an issue. Mezuzos on internal doors should not need regular checking if they are in good cases and are untouched, though it is advisable to check those on external doors that are exposed to the wind and rain every Elul.