What you are missing when you speak of the water moving though is that the water has been moving through metal all the way from the pump in the ground or lake and all the way to your faucet in many cases. If you look under the sink, you will likely find metal pipes. Whether it is brass, copper or iron, most plumbing is metal because it works so well. The only place one finds plastic is new construction in some locations where they are willing to go plastic due to it being so much cheaper than copper.
If you are even five miles from the water treatment plant, the water is likely to be in that metal for five miles so worry about it running through a valve at the tank is just wasted. Other than a model, I've never seen a plastic water tower or fire hydrant!