Facing the throttle body its on the top left side, it looks like like a round tin cylinder with a 4 pin connector. Make sure the thottle body and throttle blade are also free of carbon or sludge build up.
The IAC will not affect the idle quality. It is on the throttle body but does not have a fuel line attached to it. It is integral to the throttle body assembly itself and has a wiring connector attached to it.
Your 2000 has a very sophisticated onboard diagnostics system that detects engine misfire. If you are having a misfire that causes roughness at idle it should set a cylinder misfire code and turn on the SES light. If you are getting no SES light then it is unlikely that a misfire is causing the roughness you percieve. The IAC just controls the idle air flow to the engine to control idle speed. If there is stallling or idle instability or something the IAC might be the problem but not idle roughness. Cleaning the IAC is rarely if ever required as it is not really prone to deposit buildup or roughness.
Is this car new to you? Has the roughness recently started or always been there?? If the complaint is just a steady low frequency tremble at idle that you feel in the steering wheel and accelerator it may be perfectly normal. The STS has the 300 HP L37 engine with the more agressive camshafts which do not idle quit as smooth as an SLS with the 275 HP LD8 engine...one of the prices you pay for the extra performance.... If the idle is a complaint like this there are other camshafts that were released for the L37 and the LD8 engines of the 2000 and later design. The modified cams cam out in 2002 early in the model year and can be retrofitted to the 2000 engine if you really want them. The idle will improve with the afternumber cams. A PCM reflash is also required to make the cams work properly. See your dealer for details.
bbobnyski is 100% correct in what he is saying :coolgleam , but I have seen the IAC carbon up or just stick once in a great while causing the engine to idle too slow, if the IAC valve sticks and causes a low idle speed you can pick up some engine roughness and maybe a stalling condition, also a too low an idle speed approx 100 rpm below desired will set a P0506 dtc. If its a rough idle when first started cold, this is can also be a normal condition because of a lean calibration to meet emissions. The second one you see just below it is a throttle position (TP) sensor. I'm not sure which one you are looking that has a fuel line going to it, unless you are pehaps looking at the fuel pressure regulator installed in the fuel rail with a vacuum line attached to it. Don't forget to make sure the thottle body is clean.
Take care and listen to what bbobnyski has to say he knows his Northstars.