Since you have metal conduit which is bonded to the electrical system ground, and boxes that are also metal and bonded; you definitely need isolated ground receptacles. These are useless in plastic boxes. The only difference between a regular receptacle and iso. ground is that the device yoke in not continuous with the grounding terminal; meaning that your yoke and box cover are not bonded. I don't think much noise enters the system through device yokes, but a large metal outlet cover could possible transfer some noise. The NEC requires that all metal enclosures be grounded so this is unavoidable. Unfortunately grounding in itself is not a remedy for noise because the stray voltages see it as impedence causing current to flow.