Question: Is it true that one mustn’t use regular scissors to cut tzitzis?
Answer: The Torah (Devarim 27:5) prohibits use of metal implements in hewing out stones for building the mizbe’ach. The Mishna (Middos 3:4) teaches that one mustn’t even use a metal trowel to apply plaster to the stones. Since iron was created to form weapons which shorten lives, it is not befitting to use iron for the mizbe’ach which was created to prolong lives. The Ramban (Shemos 20:22) explains that this is why the foundational sockets were made from copper rather than iron. R’ Simcha Rabinowitz (Piskei Teshuvos 11:29) notes that tzitzis also serves to prolong lives (See Shabbos 32b).
The Shulchan Aruch (OC 11:12) writes that one should trim the tzitzis before inserting them into the begged. Nonetheless, the Rema (OC 11:4) writes that if they are too long, one can cut them even once they have been tied.
The Magen Avraham (11:18) writes that lechatchila one should avoid using a metal knife to cut them. One should rather use their teeth instead. The Shulchan Aruch Harav (OC 11:24) quotes the passuk about using iron implements, comparing tzitzis to a mizbe’ach (See Mishna Berura 11:61).
Elsewhere, the Shulchan Aruch (OC 180:5) writes that one should cover up one’s knives when reciting birkas hamazon. The Magen Avraham (180:4) explains that this serves to remind us that our tables are comparable to the mizbe’ach. He adds that it is only knives made from iron that would be problematic, but other materials would be fine. Thus, R’ Rabinowitz writes that if necessary, one can use knives or scissors made from other materials.
In conclusion, it is ideal to use something other than steel scissors to cut one’s tzitzis.