We use a combination of the 4 user defined fields which you can filter on in MR alongside of the account categories. Since the user defined fields are essentially a string you can group accounts however you see fit.
e.g. In the user defined 1 field we put items such as "Admin","Office","Courier" and in user defined 2 field we put "Revenue", "Cost" etc. This allows us to filter at different levels.
If you cannot see the user defined fields in MR, you have to edit at least one account with text within each field (i.e. Test1, test2, test3, test4) and then restart MR. If you are using datamart, you may need to refresh it to pull the fields properly.
James