The tripod grips are used mostly for drawing "lines", whether plain writing of text, or drawing contours.
However, the overhand grip is used mostly for filling areas, e.g. backgrounds. This grip keeps the tip away from the drawing surface, allowing the side of the pencil (or drawing tool in general) to contact the paper. The result is a more ...
It depends on the out come you want to get.
for instance; You can not use straight lines shading to draw a wave, or circular shading to sketch a net.
If your shading hair you will have to move from up to down
If its a wave you will have to shade in circular movements
If your shading a road from west to east you can use straight lines
If your doing net-like ...