xml:space="preserve" in programlisting, literallayout and screen

Hi there,

  I am wondering if setting the xml:space to preserve is required in
docbook 4.5. Shouldn't this value be automatically set by the DTD ?
  The documentation is a little bit confusing as this info is under
Future Change:

http://www.docbook.org/tdg/en/html/screen.html

Thanks for info !
-- 
Mathieu