Don't forget SketchUp as another free modeling package. It may not be as powerful as some of the commercial packages, or their free equivalents, but it's all I've ever needed.
I won't knock any of the other packages either, as I've only dabbled with a couple of them. OpenSCAD, though, is amazing if you can think like it needs you too.

But yes, you can design a full prototype from scratch without a printer. It's just much easier with a printer as you can find things that need fixed when it's printed, that you'd never see in CAD.