I live in NYC as well and you never need a car if you're on the island of Manhattan, but if you plan on going to any one of the 4 outer boroughs you should have a car.
Not completely true. I live in Brooklyn and have 12 subway lines within 3 blocks of my apartment, plus the LIRR. I can be in mid-town Manhattan in 15 mins., and at the beach on Long Island in less than an hour. I would never even consider owning a car. The subway is fine and I only spend $20 per week.