RE: The old age question, to leave open or closed a boiler automatic fill valve?
Do what the manufacturer of the valve says.
B&G says to leave it off at all times, and only open it for filling the boiler.
The theory is that is the boiler is installed properly, the pump is installed on the proper side of the boiler and expansion tank, and it is a leak free system, water should not need to be added. I would make sure a LWCO was installed if the Automatic Water Feed was turned off.
In my experience I would say that I see them left open 80% of the time.