If I could suggest that you create a snippet with the doc type information you need. I have several snippets prepared in the HTML Editor according to what kind of doc type I am preparing...... whether in xhtml or html in the various flavours of strict, transitional etc.
Then when I open up a new page, I just clean the existing doc information and add in the snippet with the appropriate one.
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<head>
<title></title>
<meta http-equiv="content-type" content="text/html;
charset=iso-8859-1"/>
Here are just a couple of examples but I am pretty certain you can create snippets of others you will find on
www.w3.org to suit your needs. Bill's suggestion to make your selection in the preferences options is also a good one, but sometimes you need to make other small subtle changes which is why I have my snippets to hand.