I don't know how anyone using an application and seeing a NULL value on the screen would equate the NULL value to anything other than a zero. I would default a new account setup with a zero value for the balance. I don't see how a NULL value should ever be presented to a user for a balance. To me that is a bug that must be corrected. And if you are coding the application and you get a null value for the account balance, you are going to show a zero value on the screen for the user, not a NULL. So, by allowing a NULL value for a balance to begin with in the database, you are just adding more coding to the application.

Again, if it is an aggregation or an outer join that is causing the NULL value you should either handle the Null value in the query or add code the application for those specific conditions. With the databases I design and use in applications, I never want to see a NULL value get read in by application code.