Proper way to treat rollup subcategories

Hi,

I am building a rollup table for accounts where some accounts have up to 6 categorizations and others have as little as 3. If I have a column for the 6th level of granularity subcategory, how do I treat those accounts that do not have that level of a subcategory? Should I just put in a placeholder (ex: repeat the lowest level of granularity subcategory available)? Or is it better to leave null?

Thank you!