Well, maybe. I think it depends on the project.

At the moment, I'm working interfacing a handheld device with a networking device. I quickly wrote a prototype to work out implementation details (since connectivity required using a particular protocol). That's not to say I didn't write parts of the prototype to be refactored or pulled out, but given that I had to do a fair amount of exploratory programming to determine the best implementation, much of the prototype code will be thrown away.