There should be an option at the Report file level that allows or disallows the saving of an rdl.data file along side the report.
There are cases during development when the designer will want to force the renderer to make another round-trip to the server to get fresh data each time the report is previewed.
There may also be cases where (corporate policy?) where the saving of cached data to a local hard drive is dis-allowed. As it exists now, you would need to manually search your drive for all rdl.data files to be deleted.
Also, there needs to be some documentation about the rdl.data file in BOL!