Making darker lines is part of finishing a drawing, but don't make them dark in the beginning! If you do that, it will be very difficult to erase later and the final version will look sloppy. Do you know how to do inking? It's not that difficult, you just have to be careful, and the end product looks much better in many cases. (Shading is different. That can be more difficult to pull off in ink, so I'd use the computer for shading an inked image.)