I drain the water out of the tank on my air compressor when I turn it on every day. Once the water stops blowing out, I shut off the drain valve.
I do shut off the air valve to the lines every night, and I also shut off the power to the compressor every night. I'm the only business in a residential area, it used to bug me if I heard the compressor kick on in the middle of the night. Shutting everything off solved that problem. Gene