i only had once incident when i was 16.
I took a huge bail and somehow managed to hit my mouth on my knee, then proceeded to rag doll down the hill.
When i finally stoped my braces were actually stuck in the under side of my lip, i had to rip it off, and bled everywhere. Damn Braces.
But now i have straight teeth!
For everyday skiing, i wouldn't worry about a mouth guard.