I agree with Nightcruiser. Does the twist throttle feel kind of sloppy? I resolved my idle issues by adjusting the throttle cable. In my case, there was too much "play" which rendered the idle screw adjustments useless. I added a small spacer (plastic drywall mount that I cut to about 1/4" - perfect diameter, same as the cord, I got lucky!) which effectively extended the length of the cable's outer cord. Now my throttle assembly just seems better "tighter". Plus....no idle issues..
It has also been my experience that the inner cable for the throttle has been a bit long, leaving a little play when you let off the throttle, I don't really see how that would hurt the idle or render the idle screw useless though? The slack should actually help normalize the idle and let the slide seat down on the idle screw easier I would think. Remember, the idle screw just physically bumps up against the angled section of the slide propping it open a bit, the more you screw it in the further it is propped open. Now, if your inner cable is a bit short, or has a lot of drag, it can keep pulling on the slide after you let off the throttle and prevent the slide from ever seating down on the idle screw, rendering it useless. (idle would be abnormally high)
If your cables are well lubricated and routed smoothly your throttle should snap back to home position pretty easily, and the effect on your idle from turning the handlebars should be minimized or eliminated completely. I upgraded my outer cable housings with ones that have an inner lining (from a bike store), this way the cable doesn't rub against the spiraled metal inside the cable and cause drag. I also used a few flexible "noodles" on the throttle and clutch cables, you can find them on Amazon and elsewhere. They are kind of like little goose necks that allow the cables to make bends more gracefully with less resistance.
Also, another way to get rid of that extra slack in the throttle cable is to shorten the inner cable. It can be done easily with electric soldering tools (or even a torch lighter). You just need to relocate the ball end that goes into the slide up the cable a bit. I wrap a little bit of small gauge solid core wire (like telephone wire) around the cable where you want the new ball to be, then solder it in place with regular electronic solder. Leave a ball about the size of the original ball end, you can cut/file/trim with manual tools or shape with heat. I have never had one let loose on me yet....