It wont take them down to 0 right away, but it will definately help. If your tank is still cycling then your going to get more nitrate buildup then normal. How are your fish looking? Are they sluggish, not really very active? If yes, then i would do a 30-40% water change. Figure that to be somewhere around 15-20 gallons (taking your rock/sand into consideration).
However if your fish seem to be doing dandy, swimming and eating normally then I would wait until your nitrItes are at 0, then start doing the water changes.
Edit: Ah, just reread your first question about the nitrItes going to zero, not nitrAtes lol. A PWC
will definately bring the nitrItes down some. Its better though to let your tank do that to help build up a good amount of benifical bacteria.