black water tank
I always leave the black tank gate valve closed. I dump as needed. Usually after dumping the black water, when I know I am leaving, I will close the gate valve and add fresh water to black tank to allow it to slosh around while travelling. Helps dislodge any residue. Only time I do not do this is when I have completed my last summer outing before winterizing.
As with every other poster I never heard of leaving gate valve opened.