If you don't unplug the IACV, you could adjusting the idle speed while it's still being IACV-controlled. If you adjust it to what you *think* is the stock idle speed, when the IACV shuts off, your idle will be low, or you may even stall out. You basically want to adjust the idle speed with the IACV disconnected, and no electrical load(A/C, radio, etc).