Sometimes there's no need to draw a line, you can just hint at the ends of it, and the eye will fill in the rest . also it depends on the style and how loose you want the drawing.
Examples: sink pushed to together, round forms, bending out of view, backgrounds, Sharp objects, Light or shadows depends on the effect you want, light materials,fur,small details like nose and month,eyes and more...
Make the lines a little thicker where there's a gap. where clothing stretch over
gaps between muscles or cleavages.
I don't really suggest line that much in my work, doing this has made me realized my line work is a bit to tight, and I really want to do more of this also it saves time and it looks cool.