Blah, you're fast enough.
I don't have easy access to a camera, but here's how I sand my DIY cubes.
Corners: There is one side of the cubie that was cast separate from the rest ("the top"), and most likely they're not glued together perfectly, so I sand the four sides around the top to make it smooth.
Edges: Hold on open side up with a stickers on R and D. Along the very top is a small ridge resulting from the mold. Sand off the front and back ridges.
I also sand the centers (before the caps are placed on), but if you have use store cubes, you don't have to worry about that. Just keep in mind what is rubbing against what and you'll have a good idea which surfaces need to be smoothed out.