I would never leave valves open to dry out. That also dries out the valve gaskets and shortens their life. In the summer some water is left in the tank. In the winter some RV anti-freeze is left in the tank. I do realize that some RVs have a very shallow tanks which takes a little more care in dumping. I think one thing that is most important. The more water you use when you flush the less problems you will have when you dump. I will have to admit the idiot engineer that came up with the shallow tank design probably never RV'd a day in his entire life.