10 years ago, I'd said "at least third normal form"... but today: Whatever gets the job done. When the application is not really dependent on weird queries (e.g. just a blog), screw the normal forms and design your schema to use the least number of queries for a certain task. Nobody understands three lines of code of queries with left and right joins.
On the other hand, if your bookkeeping application uses a database try to keep things as tidy as possible.
On the other hand, if your bookkeeping application uses a database try to keep things as tidy as possible.