Best way to adjust the cable without affecting idle is to do it with the car on and fully warmed up. You'll be able to hear when the cable is tightened too much since the engine will rev up a bit. Normal warm idle for a GTS 6 speed is 750-850 RPM. Normal for an auto is 700-800. If it's more or less, clean your IAC valve.
Another good way that I discovered is holding the throttle down yourself while pulling the cable tight and screwing the cable into place.
If you hold the throttle down while you tighten the wire, then it shouldn't effect your idle.