Is it acceptable to open and then close the session for an external module?

Is it acceptable to open and then close the session for an external module?

I'm writing a small reusable user manager for my projects that will use
services as facebook, persona.org for login. From the point of view of
each web, it's an include and a couple of echoes. However, it uses
$_SESSION, so I need it initialized. For that I use this code in
'include.php':
<?php
if(session_id() == '') {
if (headers_sent()) {
throw new Exception ("You must include this file before sending any
header.");
}
session_start();
$NoSession = 1; // Leave everything as it was before
}
// MAIN CODE HERE
if ($NoSession) {
session_destroy();
}
Is it a good idea to destroy the session with session_destroy() or is it
acceptable to leave the session opened?