Closing some tags is optional in HTML 5. Others are still required. I migrate a lot of sites from XHTML to HTML 5, so I leave the closing tags in place. As long as it validates with the W3C Validator, it's good to go.
You still do need to close formatting elements, such as div, span, table, tr, td, etc.
From my personal view, I always thought that coding was sloppy when closing tags were left out.
If you run this code through the W3C Validator, you will see where HTML 5 requires closing tags. I purposely omitted most of the closing tags with this test.
Code:<!DOCTYPE html> <html dir="ltr" lang="en-US"> <head> <meta charset="UTF-8"> <title>Netbuilders Test</title> <body> <div class="test"> <p> <span>this is a test <table> <tr> <td> this is another test </table>