IMHO the amount of water you carry can depend on where you are.In heavily populated area that are more temperate you could get by with a small amount of water. In extreme heat areas and less populated areas where towns can be over 60 miles apart and limited cell phone coverage you need maximum amount of water. Again IMHO a full load of water is not going to effect mileage to any great degree and short of floods excess water will not kill you but not enough can and has.
As far a filling I fill at campgrounds but I fill through a charcoal filter and always taste the water before filling.