I would recommend that rather than adding the extra record to the business object you should use the TopMostItem() option on the ComboBox itself... you can specify both value and a display value for the top most item in the list... generally "<Not Selected>" and 0, or something like that.Also, if you ever add a record to a business object manually, and you don't want that record to make the business object dirty, then you can accept the changes on the row like this:
MyBO.CurrentRow.AcceptChanges(); //-- Make sure you're on the right row...
This way, you add new rows all day log without the business object thinking it's dirty.