NUGE, twisting the cable does not change your string length at all. It does change cam rotation by making your drawlength longer. I think you know this and just stated things wrong. No biggie.

Here's a good way to tell if your rigging stretches. Once you get your cam set where you want it for rotation and drawlength just take a pencil and draw a line on the side of your cam where it passes through the limb. Measure brace height and ATA. Write these down and save them. Now if your string stretches you'll notice the marks on your cam will move (you'll see which way). You can always twist it back to where you had it.

Also take note of this idea. I do not replace string and cables all at one time. I will replace my string, shoot it a few hundred shots to get any stretch out of it. Then set your cam rotation according to those marks on the side of the cam. Then change the cable and do the same thing. You should then have setup that does not vary at all if you buy good strings.

Hope this helps.