As you are sanding along the lenght of the leading edge, you may need a few extra short strokes on the ends as they get short changed just a bit when sanding and can end up a little higher than the middle if you don't correct for this.

Sanding at a slight angle along and around the nearly finished leading edge can help get an even smooth curve. I'll also use a fairly soft medium grit sanding sponge rather than a block as you can control how much you are taking off by how much pressure you apply while also allowing the sponge to conform a bit to the curve which eliminates any flat spots.

Sighting along the leading edge in good light will reveal any high or low spots that may need to be addressed. The hardness of the balsa often may not be consistent along the entire leading edge and you may need to sand the hard areas just a bit more to keep things even.

When sanding a wing leading edge the exact shape is not critical, but you do want a smooth curve transitioning from the upper and lower surface of the ribs or leading edge sheeting and you want the stagnation point to be in the correct location as shown on the view of the airfoil section. This is what determines the front edge of the chord line and if it is too far above or below the intended point it will effect incidence and angle of attack. Also don't let the leading edge get too pointy, particularly on a trainer, as the airflow stays attached better on a wing with a slightly blunter leading edge at the low reynolds numbers involved on an R/C aircraft. Too pointy a leading edge will increase stall speed and in some cases cause a sharper stall break to occur.