There is a good reason why an airplane's leading edges are rounded. If the stab leading edge is square, the drag of the stab is higher and the air flow around that stab will be turbulent. The result is more drag and less elevator(or rudder, if it's the vertical stab) control because of the "dirty air" flowing over the control surface.

You may not notice a big difference in the flight performance of your particular plane, but, in a high-performance aerobatic plane, it can reduce the performance significantly. You should get into the habit of rounding off leading edges.

Also, the trailing edges of the control surfaces(rudder, elevator and ailerons) should never be left square, because the trailing edges have an even greater effect on drag and control, than the leading edges. They should be tapered like an airfoil, not just rounded, for best performance.<br type="_moz" />