Thats a summary of the article from Dare Obasanjo on MSDN.

Classes with Fields or Properties that Hold XML

If a class has a field or property that is an XML document or fragment, it should provide mechanisms for manipulating the property as both a string and as an XmlReader.

Methods that Accept XML Input or Return XML as Output

Methods that accept or return XML should favor returning XmlReader or XPathNavigator unless the user is expected to be able to edit the XML data, in which case XmlDocument should be used.

Converting an Object to XML

If an object wants to provide an XML representation of itself for serialization purposes, then it should use the XmlWriter if it needs more control of the XML serialization process than is provided by the XmlSerializer. If the object wants to provide an XML representation of itself that enables it to participate fully as a member of the XML world, such as allow XPath queries or XSLT transformations over the object, then it should implement the IXPathNavigable interface.