Please don't quote me on things I haven't said. The only mechanical idle speed setting is done with the engine warm, IAC valve disconnected, SPOUT connector unplugged, CANP plugged and PCV plugged. Only then is the idle speed screw used to adjust the base idle speed to 770-830 rpm. This is rarely, if ever, required on our engines.
The EEC is solely responsible for the idle speed control through the IAC valve. The only thing you can do is to properly teach the EEC the loads that it must accommodate at idle. This is done by the idle program reset procedure which I have posted enough times in the Forum that you can search on. The thought of a "tune-up" changing the idle speed says it is time to get a new mechanic or ignore the one you have.
The SHO's tachometer is not a calibrated device. Mine reads from about 900 to 1100 at idle, depending upon the ambient temperature. I know it reads high as the rpm shown during state inspections or when hooked up ot a dyno reveal the true rpms to be a bit lower. The EEC will take care of your idle speed as long as you teach it the loads.