No. When you create NearlyFreeSpeech.NET DNS for a site named www.example.com, your NearlyFreeSpeech.NET DNS is actually being set up for your registered domain, example.com. You may add as many subdomain names as you want to an existing NearlyFreeSpeech.NET DNS domain at no additional cost.
Multiple names can each point to a different site, all point to the same site, or any combination you need.