Hmm..
http://www.w3.org/TR/xhtml1/#strict says "An XML declaration is not required in all XML documents; however XHTML document authors are strongly encouraged to use XML declarations in all their documents. Such a declaration is required when the character encoding of the document is other than the default UTF-8 or UTF-16 and no encoding was determined by a higher-level protocol."
So for DokuWiki the declaration seems not to be needed, as the encoding is sent as HTTP header (higher-level protocol)....