The rule around here is to block off the entrances to the houses till you see the first Martins. Then open up for business. The Martins will deal with the sparrows.
Course the sparrows aren't protected by the 1917 Federal bird protection legislation.
In the last few years we have started getting New England Bluebirds down here. I was told how to build houses to suit them and did. I have had returning families, of increased quantity for some time now. They are beautiful and I love to watch them live their lives.