I've been told this is important. However, I didn't contact any of my POIs before sending in my applications (basically just because I got busier than I'd anticipated in August/September, and ran out of time to do the research I needed to).
Anyway, I got interviews with 4 of the 6 schools I applied to (and at the other two I was an international applicant, which of course has its own set of circumstances). So I don't think it's a catastrophe if you don't contact them, but it's probably helpful, especially if you're not sure how strong your application is.