The stylus tip shouldn't scratch the screen under normal use. Problems do occur when people (like my kids) overestimate how much force is required. The screen responds to very light input, and the stylus tip is smooth.

What I've noticed that causes significant scratching is leaving dust or grime on the screen. Going over this gunk with the stylus WILL increase the likelihood of scratching. Keep your screen clean, pay attention to the pressure you're applying, and you should be fine.