No it is not. It usually isn't necessary since a server will drop sessions automatically after 20 minutes. But you can just add the chunk of code suggested to the top of each page in an include file to make sure it happens more consistently. The fact that it isn't clearing may be a sign of an issue on the server itself clearing the folder where the sessions are stored. You may want to ask your hosting provider to confirm session files are being cleared regularly.
Did this help? Tips are appreciated...
