Crosshairs would be misleading I suppose, since people would expect that they have to get the crosshairs over the target in order to hit it. Still, you need some visual indicator of the mouse position so that people can aim the turret. Without a visual indicator it is impossible to know how you have to move the mouse in order to turn the turret (eg. if the cursor is currently close to the center you only need to make a small wrist movement to turn the turret 180, but if the mouse is toward the edge of the screen you need to make a large arm movement.)