I was intrigued by this post as I have been hearing alternating opinions for years so I phoned up a friend who is a top shot AND was a managing director at Holland and Holland for a while and is very much engaged in the business now. He said "I always close my gun by bringing the stock up to the barrels (the other way round if I am in a real hurry...) and the top lever closes of its own accord- I don't hold it open and then push it closed. I honestly don't think it makes that much difference."

Funnily enough I was always in the other school who held it open, I guess I might try the other way now as long as it is not slammed shut