Use $POST, $GET, $_SESSION, etc.
Use them! They are there for a reason. They are easy to use, and can save you hassles later on. Sure, its ququickero type $username instead of $SESSION['username'], but it's also quicker to type $SESSION['username'] than retype your resume when a security whole a cracker finds could have been avoided if you had used $_SESSION instead of the global variable name.
It is simply smart programming. At the same time, it's also helpful to know where your variables come from. Ever find yourself looking at someone else's code and wondering where the $id variable came from. Is it a form variable, or something from a session. You simply don't know.
However, you know where $POST['id'] came from, as well as knowing the difference between $SESSION['username'] and $_POST['username'].
