Best time to empty black water tank

I never have had any problem dumping the black or grey tanks whether full or quarter full. All I know is that when I get ready to leave a camp ground I do not care where the tank levels are. I want empty tanks and flushed before I put that coach on storage for any amount of time. I have read where lots of RVers have problems dumping and that the tanks do not empty all the way. Mine has a big four inch outlet and when I open that valve, it all dumps and quick followed by the grey tank.

But if I am in a camp ground, I usually dump when the black tank is around 3/4 full. No need to let it go all the way to the top. WHY!