The Datum can be located anywhere. Having the datum located in front of the nose just gives the ease of using all positive numbers when doing the calculations. Many smaller planes have the datum located somewhere within the length of the fuselage, but now you have to do your math with negative and positive numbers and that makes mistakes more prevelant, so the engineers have mostly made big planes' datums slightly infront of the nose.. .Airclues is right, having an aft loaded CG is preferable for fuel consumption, especially on long flights, it can mean a significant savings in fuel.
Hope all this helps!