I think each year is a little different, but even after the trees go dormant they still need a little moisture through the dormant season, because that is when they will continue to expand their root system. One study showed that trees could grow 80% of their new roots in fall and winter. If you are bone dry, and if the trees were planted this year, I wouldn't risk it. If they are well established and healthy, you would probably be okay to do nothing.