I usually leave both valves closed until ready to dump. The valve arrangement in the Axis-Vegas is ideal for using the gray water to back flush the black tank after dumping it. Very easy to do.
Cheap black tank flush:
After black tank dump, leave valve open, raise the dump hose a little above the valves, open the gray valve and let gray water flow into black tank (not the sewer) till you feel it stop. Close the gray valve and let hose down on ground so black tank drains. Repeat as desired. On the final flush, while the gray tank is still draining into the black tank, shut the black tank valve and drop the hose back to ground. This last action will keep debris from building up in your gate valve track seals, making the valve hard to open and close. This will also add a little gray water to your black tank, since it should always have some water and never empty before using
Save enough gray water after your final black tank drain to flush the hose.
Of course if you have one of those tank flush systems, forget about the cheap flush.