I had a piece of rubber hit me in the eye with my face shield up. Luckily I was riding shotgun because it hurt and screwed up my vision for a few minutes.
Now I always keep the shield down or at most slightly cracked for ventialtion. I've heard that leaving your face shield up is dangerous because the airbag can grab it and whip your head back. Not sure how true it is but just another consideration.
I'd prefer the full face just for debris on track days. An open face helmet is probably fine for just autox though.