I'm pretty sure I once saw the Mishna Brura talk about how it's best not to cut tzitzis with metal and that there's a custom to cut tzitzis strings with your teeth. But now, although I keep searching through O.C. Siman 11, I can't seem to find it. Can anyone come up with marei mekomos?
The Yalqut Yosef may write that it is better not, but if one needs to he may. I think the Ben Ish Hai brings the Rashash who writes to refrain from doing so.
Kaf HaHaim Palagi (Siman 10:12) writes that one should not cut the Sisit with metal because of "Lo tanif aleyhen barzel." See Kisur HaShela 53b.
Shulchan Aruch HaRav Chapter 11 section 24
It is preferable to make a point of not cutting tzitzis with a knife, recalling the command, “Do not raise iron upon them.” Instead, one should bite them off with the teeth.