1) it's not going to hurt anything, but it's unnnessary, and IMO
unsightly but some people like them.
2)its good to have a lot of water movement on the surface of the water for gas exchange. others and I tend to aim them up or spraying across the top of the water. Some other people if they have enough water movement on the surface from filter aim them down and in dead spots so stuff doesnt settle. I always put mine on the opposite side of the tank from the filter and have it spray near it to help anything floating go to it to be picked up