I too leave both valves closed till I choose to dump. Some times I dump with less in the tank if I notice an odor outside or if I'm going to be away for a couple days. I use a clear sewer hose fitting to see how clear the liquid is coming out and will flush the black tank a couple times with water till it runs clear. This helps keep the sensors working properly. I also add about five gallons of water too tank after I close the valve to help prevent the sh** mound that can build up in the tank and I add an RV chemical pack.
TV: 2013 F350, Lariat, 4x4, CC, LB, 6.7L Powerstroke
FW: 2013 Jayco Eagle Premier 361 REQS
Additions: Onan 5500W LP Generator, Surge Guard TS,
.......Roof Satellite Dish, Vent Covers, Bedroom Vent Fan,
.......Rear Reciever, 12V DC Outlets (6), 320W Solar & Controller,
.......4 Lifeline AGM Batteries (600 Ah @ 12V),
.......Stereo Upgraded, Interior Speaker Upgrade, Amp