We use a RadDiagram for what is essentially graphic-art layout. At times, subpixel positioning and extents really are correct for this use case. We have shapes correctly positioned and displaying at subpixel accuracy; however, the selection outline always snaps to a pixel.
This is particularly relevant when zoomed in on a piece of art, e.g., aligning semi-transparent edges. It looks odd to have the selection outline lie outside of the visible extents of the art. Furthermore, when moving the art by mouse, the interaction is odd in that the art appears to move smoothly while the selection outline pops from pixel to pixel. This confused our users as to what was really happening as they positioned things.
The relevant code for this is in RadDiagram.UpdateManipulationAdorner, which uses Math.Floor and Math.Ceiling to snap the line to pixel borders. This was apparently put in to solve issues of blurriness from having a, say, 1-pixel selection outline lie at a sub-pixel position. Perhaps a dependency property to control whether this snapping happens?