I try to never automatically put down the brightest brights or darkest darks. If you start out with a middle tone you can work up to the brights as well as work down to the darks. This way you can get a larger range of values. Another good thing about starting with a middle tone and building up is that you can get a variation of colors and values by allowing that middle tone to show through your layers almost like an under painting when working with traditional media. This will give your painting a more energetic and believable feel.
Most of my lines are painted freehand, but sometimes I will put down a guideline by setting my brush down at the starting point, holding down shift, and then clicking at the end point. I then go back and erase parts of the line to make the windows.
