When checking whether the submit button is pressed or not,

if I use

PHP Code:
if (isset($_POST['submit'])) 
it works fine. If I use

PHP Code:
if ($_POST['submit']) 
it gives an "Undefined index" notice, which means I need isset() for this check.

When checking whether the username and password fields are empty or not, both of the following works fine.

PHP Code:
if (isset($_POST['username']) && isset($_POST['password'])) 
PHP Code:
if ($_POST['username'] && $_POST['password']) 
which means I don't need isset() for this check.

My question is, should I always use isset() for checking variables or is it ok not to use it where it is not required?