I ride my 700x23c slick tires on my fixed gear bike in Toronto until I get a few close calls after the snow starts. Since I live and commute downtown the roads are usually well cleared and salted. Once the snow starts to dominate the landscape and you end up with icy filthy frozen piles of slush and snow on the side of the road and you're bouncing in and out of ruts, I switch to 700x32c continental city ride tires. They have a smooth centre and inverted tread outer. I find this is enough for the winter. Could I benefit from more? Sure, but most of the time in Toronto a slick tire will do. YMMV.