Right now It is possible to select 'this month' and 'last month'. Dashboards reporting about 'last month' often include reportlets about 'the month before last month'. As this is not a preset date range, the month is selected manually. When this date range is rolling, the viewers almost never see data about the intended date range.
Still, 'rolling' is definitely the option to use, as 'fixed' will force you to update the dashboard each month. Therefore I suggest a preset date range for 'month before last month', to use in reports, reportlets, dashboards and so on.