Why should the null point be obtained in the middle one-third portion of the meterbridge wire in wheatstone's network? Does contact resistance play any role in this case?

The null point should be obtained preferably at the middle one-third portion of the wire of meterbridge so as to get maximum accuracy. In this case the ratio arms are nearly equal and the end resistances of copper strips also produce the least effect. Hence the meterbridge is most sensitive.