The Shulchan Aruch Rules (OC:9) rules that one should check the strings of his Sisit each day to assure that none of the stings are torn, and prevent one from making a Beracha L’vatala (reciting a Beracha in vain). At first glance, this seems to be a mandatory Halachic requirement. However, upon examining the source of this Halacha, the Rosh (Rabbenu Asher Ben Yehiel, 1250-1327), it become clear that from the letter of the law, one may rely on the Chazaka (Status Quo) that the Sisit continue to be kosher, unless there is reason to suspect otherwise. He says that those who are “God Fearing” check their strings just to be sure. While there are those who argue that one may not rely on the Chazaka, because strings tend to break, the Aruch Hashulhan (Rav Yechiel Michel Epstein of Nevarduk, 1829-1908) strongly disagrees. He says that if that would be the case, there would never be a Halachic “Chazaka of living,” since people always eventually die.
Therefore, Hacham Ovadia, in Ye’haveh Da’at , rules that Maran did not intend that this Halacha is a bona fide obligation, but a nice practice to follow. He certainly would agree with the Mishna Berura(Rav Yisrael Meir Kagan of Radin, 1839-1933) that in the event that checking the strings would cause someone to miss the Minyan, he should not check them. That problem could be avoided if one would follows the advice of the Mishna Berura to check them at the end of Tefila, before putting them back in the bag, where they are protected.
In any event, when checking the strings, the most important place to check is the hole of the Tallit where the fringes begin. If even one string is torn there, the whole Tallit is invalid, whereas if a string is torn in the fringes themselves, it is still possible that it is kosher.
It Is not an absolute obligation to check the strings of the Sisit before donning the Tallit, although one who does so Tavo Alav Beracha (is praiseworthy).