When you point at a cube using mouse pointer, 3DTin tries to deduce the exact cube you meant to point to in 3D space. It uses the content of the scene, orientation of view and the 2D coordinates of your mouse to calculate the cube position in the scene. However as you can imagine there are limitations to this algorithm. Especially if the view is tilted in too unfavorable angle, you may mean to point to a cube in XY plane, but the algorithm thinks you are pointing at a cube in YZ plane. Such situations can lead to frustration.
To avoid such problem, I implemented a cursor cube over weekend.
Now when you are trying to add/remove/recolor a cube, 3DTin will indicate the cube position that it thinks you are pointing to, it will be shown with a cursor cube drawn in wireframe. When you click the mouse, the chosen action (add/remove/recolor) will take place on the cube position shown by the cursor cube.
I am hopeful this will help in creating sketches with more accuracy and less frustration.
Here is a short video that demonstrates the new feature.
3DTin 