Weekend warrior here.
I dump both gray and black tanks at the end of a weekend trip. I usually add a couple of gallons of water to each of my tanks (small tanks c.24g grey, c.10g black)after dumping to keep them from drying out, as well as to get a head start on 'water to solids ratio' in the black tank for my next trip.
I'm not afraid the black tank will explode should there not be enough water in the black tank before dumping. I just don't like the possibility of the inconvenience semi-moist clumps of toilet tissue might pose when dumping; i.e. clogging the black line or the stink slinky; clinging to the walls of the tank / sensors, et al.
The addition of opening the grey valve after the black just helps rinse /pre-rinse the slinky.
To each, their own.