The reason for choosing a datum forward of the aircraft is for numerical calculations, all of the moments are positive, eliminating a possible sign error. (The moment arm forward of the datum is negative, so if the datum is forward, even located forward of the nose, then all moments are positive, as you can't load forward of that datum.)
For graphical load sheets, as indicated by John, you can get a better graphical representation by choosing a different datum, and on these sheets you will see different directions of movement for, say the forward locker as compared with the rear locker. It's a pay-off between the ease and acuracy of the graphical sheet, compared to the risk of error in moving the wrong way on the "loading bar".