This really is a bit of a personal choice thing. If you are not going to use water during the trip and you have water at your destination, then why tow with the water tank full.
If you want to pull up on the side of the road and have a cup of tea during the trip, then you will have to take some water. And if you either free camp or want the option to pull up somewhere nice and stay the night, then your tanks should be full.
So planning is the key on what you really want to do and what options you want available to you. After a couple of trips, you will know.