All X- (experimental) header lines can be just written in when you edit
your message. There is no particular specified order for header lines.
Most people with such lines have their software insert them automatically.
There is, however, no good reason to clutter up the headers with such
unnecessary lines. Some newsreaders are so non-standard that you might
want to know they have made a message so you can give it special
treatment. Yarn is quite straightforward, so there is no need for
any extra header.
Header lines are scanned by all forwarding mail servers. The more
junk people stuff in there, the slower the whole net works.