I've put SSIS packages in TFS source control for years, so I know it works.  My opinion is that it probably makes the most sense to keep it in source control as opposed to the SharePoint documents folder.  While SharePoint does provide some simple versioning, it would seem best to keep all your source in one place, and TFS would be the more robust option.  It's about fitness to purpose, and using the tools for what they were designed for.  SharePoint is more of a content management system for end users, while TFS is a development and testing suite of tools.