Originally Posted by FBinNY

Many assume that a wheel off center in the seatstays means the rear triangle is asymmetrical to the centerline (assuming wheel is dished). That's the most likely cause and needs to be checked by the string method.

However there's another possibility and that's a wheel that is not lying in the plain of the frame, ie not vertical when the bike is. That's more common on new bikes because it only takes a tiny error in the heights of the two dropout slots to produce a visible error - the rim will move by 4-5 times the error in the dropouts.