This keeps the wasps on hand to control the blowflies. However, it also raises the level of the nest closer to the box opening, and may make the birds more vulnerable to attack by predators. The Cornell Laboratory of Ornithology is currently studying the nesting of bluebirds and other cavity nesters to see what's best. On the trails Joe asked about more than one box. In fact, series of bluebird boxes, called bluebird trails, are increasingly popular arrangements in open areas, such as at the edges of fields.
Some people maintain trails with dozens of boxes.
If you are planning more than one box, place each at least 300 feet from the nearest other box. And consider techniques to discourage predators. One is a baffle on the box pole. Called predator guards, these devices can be purchased at a birding supply store or created at home. One simple technique to keep climbers off is to put axle grease on the pole that supports the house.