You COULD shave with nothing but cold water, but why would you?
Any soap is better than no soap, but shave soaps are designed with specific traits (latherability, slickness, cushion, etc) that make the shave process more comfortable. Bar soaps are designed to remove dirt and oil from skin, so they're less than optimal to shave with except in a pinch.
Thanks guy. I *thought* that I'd heard of someone here on B&B using bar soap but I could be wrong.