can any one tell me, Why it is always recommended to ground neutral at only one point in a voltage level? Is it related to flow of third harmonic currents? if so how?

With regard to commercial building wiring in 3-phase, 4-wire systems, it has to do with grounding safety and not harmonic current flow. The neutral conductor is "bonded" to ground at a single location so that normal unbalanced (neutral) current will not flow in the ground conductor and in the metal frames/enclosures of electrical equipment that are connected to ground. The purpose of the ground conductor is to provide a low impedance path for ground fault current to return to the source and have sufficient magnitude to quickly trip overcurrent protective devices. This minimizes the touch voltage (magnitude and duration) of the frames of equipment/enclosures that a person might be subjected to during ground fault conditions. If the neutral conductor was grounded at multiple locations, then touch voltages would be present during normal unbalanced conditions and this could produce shock hazards to a person touching the grounded frames/enclosures of equipment. The purpose of a neutral conductor is to provide a return path for unbalanced current and the connection single-phase to neutral loads.