In drawing, there are no rules. If you make some, you will end up having to break them. I have found that here are principles that govern all approaches to drawing. Drawing is basically a visual search of the forms we are seeing and recording on paper what we discover in that search.
Whatever kind of line you use to record those discoveries is highly personal, but consciously or not every artist when drawing from life is searching out four visual relationships:
1. spacial position. Where is this point(where the contour changes direction, a shape terminates, etc) in relations to some other point?
2. Size relationship- How long is this section compared to that one, (The width of an eye compared to the distance between them, the width of an arm compared to the space between it an the door or whatever.)
3. Angle relationships - What is the angle of this section of a form compared to vertical or horizontal, or to an adjacent angle?
4. And finally, when all of the shapes have been placed, Value Relationships - How dark is this shape compared to that one or to white or to black?
The contour is simply the edge of a form. Some people use short choppy strokes to define the forms, but I find that for me that shatters my vision. I try to place a contour in as few lines as possible. It means first looking at where the starting point is in relationship to a nearby one, and where I will end, then stating the edge by pulling the line between those two points. That is contour line drawing, and is indispensable to painting.
Some grand examples of contour line drawing are to be found in the works of Hokusai, Rico Lebrun, Lok (here on flickr), and of course Charles Reid. But if you look carefully at the drawings of Rembrandt, Degas, Ingres, Delacroix, etc. you will find they all used contour lines. In many cases the value rendering hid the initial lighter contour lines, but in the unfinished areas there they are-beautifully pulled, confident, contour lines.