Sketching in 3d is something not something I seen done allot. But I have bean tempted a couple of times my self. Because sometimes sketching up some reference pictures and the like are not explorative enough. Figuring out the volume and shape of something made up is something I think 3d or clay is good for. If you don't have clay, use 3d. Box modeling or artisan is the closest thing I can think of next to clay. Thoughs are not my normal tools for when I know what I'm after though.