Always wear a helmet, even if you don't think it matters.
This is not an issue of legality. Riding a bike without a helmet is perfectly legal for adults in NYC (for now, anyway) and is purely a matter of personal preference. Is a helmet better than no helmet? Yes, if you fall and hit your head in exactly the right way, not so much if you get hit by a bus. But if you DO get hit by a bus and are not wearing a helmet, rest assured that fact will be in the first line of the New York Post story on your untimely demise. Don't give them that chance.