I use whatever is handy. Usually dental floss, but I've also used braided fastflight, dacron serving, strands from an old bowstring (ff and B-50), upholstery thread....

I think my favorite is dental floss, because it's thin enough to wrap down into the serving and won't slip. Of course, you can't adjust it either, so you have to replace it if it needs adjusting. That's usually not a problem though. Unless you've had to twist the living snot out of the string the nock point won't move significantly.

One thing I do after twisting the string is to restring the bow, then twist the string under tension, about 5-6 twists, then slowlyslacken my grip and on the stringand let all those twists unwind.My theory isthat helps immediately distribute the twists you put in the string evenly over it's whole length instead of keeping them on one end until they finally distribute on their own. That keeps the nock location constant.

Another of my little tricks... I tune the bow and find the right nock locationusing a brass nock. Then I measure it and replace the brass one with a tied nock.