What separates good art from great art is a sense of depth to the image. Shading adds tangibility to the image, and makes it look more natural.
The Lasso Tool is typically used for selecting irregular sections of an image. You can, however, use it as a drawing tool.
When a comic is going to be inked and coloured on a computer, it is important to bear this in mind while drawing the original artwork. Often all that will be needed is a basic sketch, like any other form of sketching to ink but, unless you’re prepared to spend time adjusting the scanning quality and contrasts, etc., getting only the details out that you actually need can be tricky.