I didn't contact a single professor, and got into good places. I know people who contacted profs, and got into those places, and people who tried that, and didn't get in. In my opinion it's a complete variable which depends on:
1) The school. Department culture and approaches to applicants varies incredibly.
2) The professor. See above.
3) Your email. Be sure you're asking a question that you cannot find the answer to by googling it (at all), or talk about how your research/skills might interface with their work. You could ask if they are looking to take graduate students in the coming year. Just make sure your email is appropriate and makes you look BETTER than you would have looked had you not contacted people -- otherwise contacting professors can actually hurt your application.
In short, it isn't necessary, but if you do it right, it can either help or not hurt, in my experience.