These lines are a feature that Paint.net is sadly lacking; I propose we start developing them so they can be added in a future release.
If you don't understand / know what I am talking about, then imagine being able to make a reference line that is not rendered onto the image, that is dragged from the rulers on the top or side. These horizontal or vertical lines would be snap-to points for all of the tools you have. This would let you make a horizon line, and make spaced vertical lines so you could have boxes evenly spaced, that are level with each other.
