I always leave both valves closed. When I am camping with sewer hook ups I dump the black at 1/2 or 2/3 rds then flush the hose out with the grey. The reason you do not want to leave the black open all the time is the liquid will all run out and leave the solids in place.....
09 1500 Suburban 4x4 LT2 with factory tow mirrors
2012 Jayco Jay Flight 26BH
Elite package, cold weather package, ladder, electric stabs, awning, and tongue
Two 24" 1080p led tv's
Prodigy p3 brake controller
2 eu2000i Honda's ( one companion )
All lights converted to LEDs