SketchUp contains a few Soften Edges options allowing you to manipulate the use of softening within your model.
Note: Edges that are shared by three or more faces cannot be softened. The following image shows an edge shared by three faces.
Angle between normals slider
Use Angle between normals slider to set the maximum size of all angles that will be smoothed or softened. The higher the setting, the more angles you are likely to smooth or soften with a smooth or soften command. Consequently, the higher the setting the more performance might degrade.
Smooth normals checkbox
Selecting the Smooth normals checkbox smoothes any edges, essentially spreading color and texture over edges, to make the edges appear smooth.
Soften coplanar checkbox
Selecting the Soften coplanar checkbox softens edges, essentially deleting edges, between coplanar surfaces.
Create a three dimensional multi-sided polygon (with more than 5 sides) and experiment with these options to become familiar with softening and smoothing models. For example, create a 6-sided polygon, expand the polygon using the Push/Pull tool, select the entire 3D polygon, click on Smooth Normals, and move the Angle Between Normals to 61 degrees. Notice that each edge of the polygon looks smooth because the angle between each normal is 60 degrees and, therefore, can be smoothed. Now move the slider to 59 degrees. Notice that the sides no longer look smoothed because the Angle Between Normals only operates on angles below or equal to 59 degrees. Experiment further by turning on the Soften Coplanar option to see edges appear and disappear.