Why should photographers always put metadata attached to their digital images? Because a.) you should anyway and b.) it tells anyone where the image is from and what the precise usage is. Most art buyers/art producers and photo editors know that the metadata is attached to the image. It's as simple as pulling up the file info. But why don't more photographers attach metadata?
There's a great article all about metadata in ASMP's fall 2007 bulletin - Basic Metadata: A Photographer's Best Friend. In this digital age it's important and critical. Most digital cameras record the technical info but in post-production photographers should embed the personal aspects of that image with copyright, contact info, client, usage, etc. The usage is particularly important. Clients are always looking to use the imagery for all sorts of print and web use. The agency that does the print work is not necessarily the same that does the web. So if the client passes the image to the web agency the information should be attached to the image to notify if they can even use it for the web and if so, when does it expire. It would be great if there were one digital asset librarian trafficking and being responsible for the image. But as we all know from the time of the image capture to the actual production, it passes through many hands.
The ASMP article points out that there are templates in programs such as Photo Mechanic, Adobe Photoshop, Adobe Lightroom, and Apple's Aperture. I have personally used Adobe Bridge to input the agency's information into the image (info like client, job number, cost, usage, and expiration date). As an art buyer I love when I open up a file and the photographer has tagged everything because that way I know s/he's aware of the usage that was negotiated and can track the image if necessary.
This simple step ensures that all the pertinent information goes with the image through all its travels. Developing a good metadata workflow is like remembering to floss your teeth, it's a good habit to get into.
An exceptionally useful site is the Universal Photographic Digital Imaging Guidelines or UPDIG