I think , ideally, I would prefer to have the grey tank full or close so as to flush out the system when dumping. I don't think it matters so much with the black as long as it goes first. Unfortunately, it's seldom a perfect world. Rather than risk filling up part way through the next stay I will often go ahead and dump when I'm leaving a campground if we have 2 or 3 days worth in the tanks. That also saves some weight when on the road. However, we try to avoid solids in the black tank if not too inconvenient and that saves a lot of worry. We also put a lot of dish water down the stool to clean, dilute and create more space.