Using SharePoint SQL directly is not supported by Microsoft, so for example the internal structure may change in an update, which may break your query and you will be left to fix it alone. Any updates to the database(s) and Microsoft will no longer provide support for your farm.
You can easily use SharePoint list data in SQL Server Reporting Services by using the SharePoint List Data Extension. You point it towards the list web services, specify the fields and away you go.
Here is a technet articletechnet article about using the SharePoint List Data Extension in SSRS: http://technet.microsoft.com/en-us/library/ee633650.aspx