When you edit a UTF-8 file in Visual Studio, it adds the byte order mark (BOM) sequence to the beginning of the file. You can select an encoding manually, but you need to do it each time you reopen the file.
Fix File Encoding automatically detects when a UTF-8 file is opened in Visual Studio and sets its encoding to UTF-8 without signature. If you don’t edit the file, it remains unmodified. If you edit the file, it will be saved without the BOM. You can configure which files to encode based on the file path and the file extension (by default, only .html files are protected from Visual Studio).
You can download the free Fix File Encoding extension for Visual Studio 2010 and Visual Studio 2012 from the Fix File Encoding homepage.