This has been mentioned here before, I think (you know that ole brain
We have always been told and practice the following: You should always KEEP all valves shut, even the gray water (except for that washer situation). By leaving them open, the debris and soap scum collect on your sensors, dry and turn to a concrete like crust.
Now, if you do have the washer/dryer, you gotta do what you gotta do to not flood Montana. So, the keep the valves CLOSED rule would be for any time other than when you are running the washer.
We go 2 or 3 days, then dump the gray and the galley tanks. Now, if I am grooming dogs, after I do two of them, I have Al pull the valves, empty, shut and I do two more.
Like Steve, we just kinda know after all these years how often to dump the gray and galley, but, that said, the other day I did manage to fill up the galley, but I had washed dogs and he forgot to pull the valve. I had to wash a few dishes over again, not a big deal, eh???
I cannot remember the last time we even looked at those little red lights on the command post, oh, are they still red???? or are they yellow????