I say, typically, you should use the full standard method of delcaring your PHP code sections as suggested above.
BUT, the following guidelines can help anyone who has this question:
If you plan on sharing code: use the full declaration <?php ... ?>
If you will not share and have total control over the server where it will run: do whatever you like short or long
If you will not share and you DON'T have total control over the server where it will run: use the full declaration <?php ... ?>

So following those guides, only people writing code for an internal prorietary system with full control over server environment should use short tags for declaring php code sections, ALL others should just use the full <?php declaration ;-)