What kind of wiring system do you have in your house/apt?
Have you confirmed that the grounding terminal in your outlets is actually connected to ground?
If it is, and you have metal boxes, I would almost rather ground to a box itself than through a cord, where it relies on a wired connection. You could attach a small piece of copper pipe hanger strap behind the mounting strap of the receptacle, (using the receptacle mounting screws, and trimmed so it doesn't contact anything else, of course), and leave a tail hanging out from under the cover plate to attach your grounding clip to.
