I bought the
Hori DS Lite screen protector. You have to be real patient when you put it on or you'llget bubbles and dust under it. I really like it and it doesn't affect the screens look in anyway.
Take note that while people tell you closing the DSL will be enough the screens, the problem is "stuff" will get on the screens when you're playing w/ the DSL and when you clean the screens, no matter how careful you are, they will get scratched. With the screen protector, you can easily wipe the "stuff" off w/o scratching the actual screen.
In the end it's really up to you. I admit, I'm really anal about my gadgets and tend to keep them in prestine condition from the day I purchased them to the day I sell them.