Jim,, I have been driving this House for 8 years, in that time there was one Propane fill I did not witness.. ALL the others the pump jockey opened the visual liquid overfill indicator valve, ALL of them.
I have seen one portable filled where they did NOT open the valve.. It was not mine.
In the many years of trailering before I got this A.. They always opened the valve on the tank.. When I happened to be sitting at a review of the final examp for propane pump jockey certification (In Michigan at least they are certified before they pump. Also in South Carolina) they specifically had questions about that valve (Which is where I first heard it's proper name). and it is to be OPENED.
So I don't know where you are getting tanks filled. But I've seen it used for a lot of years. Going all the way back to when My dad was a Pump Tender on an oil well and I'd ride with the propane driver to open and close gates for him and direct him to the well which was just BEHIND our property (And accessed via our land).