Should i buy a screen protector for the DS?

It may be a factory defect with my stylus (Although its happened to two different units) or it may be the dust in the atmosphere or something (I live in the southwest US where its more dry and dusty) or maybe I'm just heavy handed or I play my DS more than some do. *shrug* Whatever the reason, it happens with me.

*lol* Thx for clearing up about the glass cleaner. It definately didn't sound like a good idea mixing ammonia and plastic.