Most of the quality commecial bowstrings, like Winners Choice, Zebra, and Barracuda, are packaged with the correct number of twists.
What I do when installing a new string is to lay it out and measure it first before putting it on my bow. I am careful not to add or take away twists when doing this. I then put it on and check the ATA and brace height to make sure they are close to specs. If it is, I set up a rough level nock point and center shot and then shoot about 200 shots through it. I do this before fine tuning with the new string. I then check ATA and brace height again to check for string stretch. There is very little, if any, with today's quality strings. When it all checks out I then proceed to fine tuning my bow.
Also, if you shoot with a peep, after the 200 shots, you may need to add or take away 1-2 twists to get it to align correctly with your eye, and that's okay.
I'm sure there are others here that do things different, but this is what I do.