Only use Strict Doctype headers when producing HTML 4.01 and XHTML 1.0 web pages. If you use Transitional Doctypes you are incorporating deprecated Markup and your pages may render in Quirks mode thereby inviting problems.

Remember that if you serve XHTML as content (MIME) type text/html (as 95%+ presently is) you are actually only producing HTML pages - if you wish to produce them as true XHTML (with XML functionality) pages you need to serve them as application/xhtml+xml - see Problems Serving XHTML 1.0 Web Pages