Always filling to the top will help but not by much. Within the first few minutes of filling to the top, the top of the filters will be exposed to air and stop contributing to the effort while bottom of the filters will continue to filter. The bottom of the filters won't be exposed to air until nearly all of the water is gone from the top chamber which is probably a couple of hours of filtering. So the bottom of the filter will filter the entire time but the top only for a few minutes. I have noticed that after only a few months the bottoms of my filters appear to be plugged and a couple of inches of water sits in the top chamber pretty much all of the time. I am not complaining. The water coming out of these things is great and I have no problem replacing plugged filters. When I do place my next order, I will buy an extra set.
2Hut8