If "slightest movement" lets the nut back off, then it wasn't tight enough. I've been in nuclear plants, and they require antiseize LUBRICANT on flange bolts that they definitely don't want to come apart by accident. They are often subjected to vibrations for many years. That said, if the threads are plastic, and clean, there's not really any NEED for lubricant. I just like it because it allows me to get the same pressure on the rubber gasket with less turning pressure on the nut.