Whether you declare the encoding by passing information alongside the document in the HTTP header, or inside the document itself, you should always ensure that the encoding is declared. If you don't do this, the chances are high that your document will be incorrectly rendered.

If there is a chance that your documents will be read from or saved to disk, CD, etc., then you should always declare the encoding inside the document. (This does not rule out also declaring it in the HTTP information provided by the server.)