I don't mean to imply there is only one correct way, however there is definitely an incorrect way - and covering the purge valve with your hand is definitely incorrect. Anal? Humm, sure. But if you make yourself learn to do something one way every time then when there is an emergency you can react without thinking.
Any yes - every single person I've checked out has covered their purge valve with their hand - when placing the reg in their mouth (no big deal) or when donating (big deal) during OOA. And of course it's not their fault - they were trained that way.
BTW - There have been some funny as hell posts on this thread. However to be an instructor and to say the above quote is kinda irresponsible.
John