Once you have shot several arrow with a loop you shouldn't need to serve or use a nock to keep it from moving. They aren't moving if you tied it right (trust me, I have tried moving them, and they don't).

My biggest problem is that they stretch and the loop becomes longer. Even when I tie them with just enough room to get my caliper jaw in, they still stretch. I replace my loop every so often when they get too long. Might have to look at a loop material that doesn't stretch as much.