Leaving a washing machine's valves open all the time greatly increases the chance of a catastrophic hose burst. This is why professionals suggest shutting them off when a machine is not in use, but most people don't bother.
Fortunately, there are devices that will do it for you. The Intelliflow (about $150) senses when the washer is turned on and automatically opens the water-supply valves. At the end of the cycle, it shuts them down. A floor mounted sensor also closes the valves if it detects water pooling around the washer's base.