The layout grid in RadDiagram is a little lacking and it would be a lot more powerful if it supported a similar layout grid to Adobe Illustrator. The features of interest are:

Grid Subdivisions - You specify the gridsize as currently, say 8px, but then you can also specify a number of subdivisions, say 2. This will split the 8x8px grid cell into four 4x4px grid cells, and allow snapping to the 4x4px grid cells.

Grid to back/front - allow the grid to be overlaid on top or behind the diagram so that you can optionally see the grid all the time.

See attached diagram.

Thanks

Reason for closing / declining

The mentioned cells can be controlled with attached properties of the BackgroundGrid and BackgroundPageGrid.
Please check out these articles:
http://docs.telerik.com/devtools/wpf/controls/raddiagram/styles-and-templates/styling-appearance.html 
http://docs.telerik.com/devtools/wpf/controls/raddiagram/features/printing#pages-preview-canvas 

To move these grids on top of the shapes you need to edit the default template of the RadDiagram and reorder the Grids in XAML.