I was struggling with the same problem - in the end the solution seemed fairly simple - I just set the grid so that I could snap to half pixel resolution. Then I can place a one pixel line in the middle of a pixel space (at 0.5 pixel) or a two pixel line on a pixel boundary.
I am only working on small images, so I have the grid spacing set to 1px with 2 divisions. I can imagine this might be less workable for larger images, but if you are looking for pixel accuracy then I guess you will be zoomed well in and a 0.5 pixel grid will hopefully be workable.
