Most of my design sketches are built completely in 3d CAD using Vectorworks and Cinema4d. Not only does this produce an accurate scale representation of the environment, it also assists in the quick conversion to design drafting. Of course there's the usual amount of Photoshop tweaking and trickery, to achieve the overall feeling of the design.
The sketches are built in order to be edited quickly and to work collaboratively with the client. It's a careful balance of being artistic while maintaining useful functionality. The highest compliment is when a client sees the finished scenery and says "it looks just like the sketch!"