Most of the places that I camp don't have water. I fill my water tanks with 80 gallons when I leave for camping no matter how far that I'm going. This summer I went to Montana and I left with 80 gallons of water and had some water left when I got back home two weeks later. If you know that you will have a good, clean water supply then by all means don't fill all the way up. For me, I fill up because in Arizona, water is life.
Also, I don't notice any difference in mpg carrying the extra 660 pounds. I made my trailer to carry that much water and I have room to add 30 more gallons. So far, 80 gallons has been plenty.