Once the BOM is created, you can easily see which parts and assemblies do not have certain types of attributes or values and then write them directly into the BOM cell.
All the values written in the cells (which are actually the properties and customdata of the parts/assemblies) are added directly to each object, so there is no save button.
In the upper right part of the window there is a tab called BOM; there you can see all properties of the selected part/assembly.
Double-click another row in the BOM or select another part of the tree to see all properties.
Changes made in the BOM grid are reflected in the BOM tab.
Double-clicking a field in the BOM updates and selects the item in the Scene Browser.