Thats what we always do is leave the BLACK WATER closed until we dump. I like to dump when the black tank is at least 3/4 full then we get a full force of water flow to help clean it out. We always dump the black first then the grey last which cleans the hoses best for us If we left it open at the camp site all the time it would only drain with a small water flow force and start building up the dreaded POOP PYRAMID... You don't want that believe me.
BUMPYROAD - I have heard the POOP PYRAMID forms from the top down in Australia... Everything must be upside-down there...
Roy Ken