As a matter of practice I close them when I return the Safari to storage. I would rather not have the system under pressure while its in storage.
You have a dual regulator which means once one tank goes empty, and both valves are in the opened position, the other tank will come on line.
Personally I normally keep one valve closed when operational unless I see that I'm starting to get low. I would hate to have to get up one cold night to open the other valve if the operational tank went dry.
Since the Safari has no inside gauge, if I kept both valves open, which will allow the automatic switchover, and don't check when I should, I could run the other dry also. Normally once a tank goes dry, I get it refilled right away or once I get home.
Usually you can tell when you are getting low in a tank by looking at the little glass indicator on the regulator which is normally green. It will slowly be showing more red as the tank gets low. Another way of checking is on a damp day when your furnace or water heater is in use. The liquid propane changing to a gas within the tank will condense moisture on the tank which will show you the level of the liquid. Finally when the tank gets low, the smell of the chemical used to give propane its telltale smell will become noticable (at least I can smell it) when you have an appliance operating.
'04 Classic 30' S.O.,'03 GMC Savana 2500,'08 Vespa GTS 250