You can buy screen protectors, but you don't need them. I had one but eventually removed it, haven't actually scratched my screen in the year I've had mine. Just make sure your stylus doesn't have any sort of sharp edge on it and you should be good. As far as dead pixels go, my current one has one, my old one didn't have any. My friend replaced his when he got a dead pixel, I've never bothered because I can't really see it.
Just remember, for the most part, your bottom screen is your most important screen. Make sure it doesn't get scratched and doesn't have dead pixels. The top screen is often used for stuff like maps, radars, notes, inventory, etc. So it's a 2nd glance screen. The top screen becomes more important in games where they have you hold the DS like a book. So both screens are important, but the bottom doubly so.