After drawing with the grease pencil you can select parts of the line like you would a vertex/vertices ( the line tool would only have two points ) and can use the number pad/arrow keys and snap tools to move them around as well as using the keys X, Y and Z during movement to restraint. For exact positioning of the cursor i use an empty object and move the cursor to the selected empty ( or part of model ) for specific tweaks.
You can use shift+s for some control over the cursor.
EDIT - You can use left control to use snap temporarily instead of having it on all the time ( the magnet icon ).