With the new Account Breakdown feature in Partner Center, you can determine which accounts had a given product active in a given month.
To view your account breakdown, go to Partner Center > Administration > Billing > Account Breakdown.
You can also click on a given product in the Estimated Usage tab to filter that product in the Account Breakdown tab for the current month.
This feature is available to all Partner Center admins that have access to billing reports. To manage the permissions of your admins, go to Partner Center > Administration > Admin Management and follow these instructions.