Another method is to use the follow me tool. Knowing that SU has issues with small facets, I used a 100' x 100' rectangle as the base geometry to create a cube. Didn't find any holes, but close up shows there still needs to be some line hiding if you're after perfection. A 10" x 10" cube after follow me has the annoying little holes in the corners. I upped the arc segments on the smaller cube to 24 vs 12 on the larger cube.
A few of these little boxes and 2 gigs of RAM with a 2 year old video card shows some strain.
Same technique works with other various shapes to end up with a variety of radius cornered shapes, some requiring more cleanup than others.