Mike,
I believe the biggest reason that sticker is there is because if the termination valve is closed and one uses the tank flush inlet, and walks away to do something else and forgets he has water going in the flush inlet, then a real problem arises inside the RV when the black tank fills up. I have seen several cases where this has happened.....all due to forgetfulness.
I always close my dump valve about a minute before shutting off the tank flush valve because I like to have a couple gallons of fresh water swishing around the freshly dumped black tank to ensure it is clean....plus water down in the dump valve area ensures your rubber valve seals don't dry out.
