For me it depends what Im doing...if it sketching then it`s back and forth, if I`m working on something from a photograph, then I pay a lot of attention to the lines as I`m drawing them to get proportions right then add shading and blending. In art school we did the drawing of the left hand using the right only we were not allowed to look at the paper. Someone once said of...think it was Monet...he drew with his eyes and saw with his hands, don`t ask me to explain that but there is no one correct way, you have to work at it and find what works best for you.