The relief valve should normally be closed, but should be opened now and again to make sure it operates freely, if needed. I use the relief a few times each year to drain the water heater, if the camper sits for more than two weeks. (The water can get pretty skunky just sitting in it!) Using it for draining also exercises it, so it's a win-win.
2015 Jay Flight 32RLDS Elite
2012 Chevy 2500HD Crew Cab LTZ (6.6L Duramax/Allison)
Equalizer Hitch (1200# bars)