When "city water" was available, I hooked up to it. Lately, I have begun to just used the water on board. The reason is this. By using the water in the tank, I will be able to keep it fresh. When the fresh water tank is emptied, I will just put more in via the "city" water.
And as an added benefit, I will not have to worry about a flooded trailer should a fitting fail. I know the city water faucet should be cut off when ever I'm away from the trailer, but I tend to just forget it there. I'm much more likely to remember to cut off the water pump, which I try to do when ever I don't need water.
I will usually travel with about 1/3 to 1/2 full fresh water tanks. My trailer can carry 84 gallons of fresh water. I have yet to need that much water before I'm at a place where I can add "city" water.