Most of my design renderings 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. There is also a certain 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 or a producer sees the finished scenery and says “it looks just like the rendering!”