Why You Should Include an XML Declaration
http://www.informit.com/articles/article.asp?p=102605&redir=1

Harold, Elliotte Rusty
InformIT
2004

Abstract:

Although XML declarations are optional, every XML document should have one. An XML declaration helps both human users and automated software identify the document as XML. It identifies the version of XML in use, specifies the character encoding, and can even help optimize the parsing. Most importantly, it's a crucial clue that what you're reading is in fact an XML document in environments where file type information is unavailable or unreliable.