Content transfer encoding 7 bit necessary?

[digitalecartoons]

$email_message .= "This is a multi-part message in MIME format.\n\n" . 
                    "--{$mime_boundary}\n" . 
                    "Content-Type:text/plain; charset=\"iso-8859-1\"\n" . 
                    "Content-Transfer-Encoding: 7bit\n\n" . 
                    $email_txt . "\n".
                    "--{$mime_boundary}\n" . 
                    "Content-Type:text/html; charset=\"iso-8859-1\"\n" . 
                    "Content-Transfer-Encoding: 7bit\n\n" . 
                    $email_htm ;

 

This is part of a html mail with alternative text version. I often see this Content-Transfer-Encoding: 7bit line in scripts like these and I'm wondering if it's necessary since 7bit is default? So why do a lot of people put it in?

[digitalecartoons]

Could I leave it out (since as far as I know 7 bit is default), or is there some reason why that line is often present in html mail scripts?

Here's an article by Kevin Yank about mixed html/text php mailing which also contains several 'content-transfer-encoding: 7-bit' lines:

http://www.sitepoint.com/article/advanced-email-php/4