3D wall cleanup is substantially improved in AC10. It's still not perfect, but many more junctions that should cleanup actually do.
Example: This tip should be workable in AC9, but it just doesn't clean up. In 10 it does, so here goes.
Instead of using a cornerboard object, use a polygon wall. Use one wall for all the stories. That is, the base is perhaps somewhere around zero and the top is up around 20'. Set the Show on to 'Automatic' and the Floor Plan Display to 'Projected'.
If you have band boards, they should meet the ends of the polywall, rather than forming a corner themselves.
You can place as many band walls as you need, on whatever story, and they will all clean up. With the object, they don't.
The wall also has the advantage of being stretchable in section. Can't do that in 9 either. (As of the writing, there's a bug in 10 which prevents vertically stretching polywalls in 3D. That can't last.)