That is correct--some of the twists can work their way out. You'd have to make the first loop standing up for the bundles to hang freely though.
Counting the twists you take out will get you close--it's the best way I know of to get a good idea of how many back-twists to put in the first time. You don't have to be dead on the money to get a nice, round string.
I look at an individual bundle after I make the second loop, but before twisting up the "body" of the string. If it has no twist in it (or very little), it's good. If not, you can tell how much you need to adjust.
Chad