I always hookup the sewer hose when hooking up everything else (assuming we have full hookup). Setup once and be done. If we are camping short term, then both valves stay closed. Tanks are dumped when full. With a family of 4, we usually fill the grey tank everyday 2 to 3 days and fill the black tank every 7 days. When we camp seasonally, the grey tank valve is open all the time and dump the black tank when needed. When dumping the black tank, I fill it afterwards with the in-tank rinser, as many as 4 times to make sure the tank is cleaned of debris and the hose is cleaned as well.
Camping seasonally, we see the weekend warriors do all kinds of strange things. Even saw someone take a hammer to one of his awning's arms trying to retract it in a storm. I went to help him with his awning during the storm, then showed him how to do it properly after the storm was over. Perhaps you could educate these people as well and maybe you won't see it happen so often. I for one have never watched someone else setup camp so intently that I knew they were opening the black tank valve and leaving it open.
-Michael