Depends on the person, I think. There's a guy a play with regularly, Frank. When we start a set, he always tosses the coin, I call it in the air, and it lands... then he asks, "What is it?" Without glasses on, the man can't see to save his life. All the balls look fuzzy to him, but they manage to find a pocket regardless.
Me on the other hand, I don't play as well when I my vision is uncorrected (glasses/contacts). But surprisingly, I don't play too terribly when I don't have them in. I'm very near-sighted and can only focus 8-10 inches away from my eyes. Everything after that is a blur. When I first tried shooting without contacts/glasses, I was surprised that I was able to pocket balls decently without being able to see. So much of my game is based around angles and "feel" anyway, so I never have used a contact point to focus on when lining up or stroking.