If you have cooling then yes you should leave the lever open. If you have no cooling then the furnace doesn't run in the summer anyway and it will not hurt to leave it open.
Now, if you hve cooling and you are in a two story home, then I would half close the downstairs levers and fully open the upstairs levers becuase the two story home is harder to cool the upstairs. In the winter do the opposite...especially do this if all the bedrooms are upstairs (say in a colonial-typer home). Hope that helps.
