As stated earlier, one reason to keep the valves closed is sewer gas. But another really important factor is if you keep the black tank open the fluids will drain as you flush and the solids will stay behind, build up, and harden. There needs to be lots of fluid in the black tank to push the solids out.
Generally what I do is wait until the black tank is more than 1/3 but less than 2/3 full. I empty the black tank first. Wait until the hose has drained. Then I close the black tank valve and pen the gray to flush out the sewer hose.
Generally the gray tank will be about 2/3 full and the black between 1/3 and 2/3 when I dump the tanks. After dumping I add a Walex Bio Black tank treatment pod and some water to the black tank after EVERY emptying.
Here is a link to my latest version of the manual. You can download as a MS Word file, or use the menu and download as a .pdf file.