I keep mine closed after doing my annual tank maintenance (prior to winterizing), and keep the pink that sloshes down the drains in the tank to keep the seals from drying out.
Annual tank maintenance is a thorough flush, add 1 pint of Unique RV-Digest-It to each tank, fill to top, let sit 1 week (72 hours minimum), flush again, close valves, then winterize camper.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW