 <p>My answer would be "Always."</p> <p>It's the emerging standard for categorizing all forms of information on the web. </p> <p>Raven Tools (no affiliation) has a schema.org microdata generator that's a good place to start:</p> <p><a href="http://schema-creator.org/product.php" rel="nofollow">http://schema-creator.org/product.php</a></p> <p>They have a couple stock schema templates on that page (look on the left column).</p>