First if you form an LLC or Incorporate, you don't need to file a DBA. I.e., you don't need to register your business name because filing an LLC or Corp. takes care of that.
All businesses that use a trade name need to register the trade name unless the business is a sole owner using his own personal name (e.g. Scott Smith is doing business as "Scott Smith Widjets," does not need to file a dba). Another exception is a corporation or LLC using their legal name to do business.
For example, Coca Cola Corporation using their name to do business does not need to file a DBA under the corporation. However, if using a name such as "Desanie," water or any name other than "Coca Cola," the corporation needs to file a DBA.