I would say if it’s an organic shape or there is potential for form development then always start with sketching.
Another good example of when to use cad and a quick rendering is IF your concept is a basic geometric shape (think consumer electronics) and you want to convey the subtleties of a precise monolithic shape in a high gloss finish or with a play of 2 materials. It would be easier to model it and render it than to try and sketch and render using markets and pencils. You can do this and then use Photoshop or Sketchbook Pro to add buttons or partlines over the rendering, etc.
The times I do this I always go in and add details digitally to give it that “sketch” look. If you leave it untouched then the client may think it’s a finished concept.