A small knock and i got a small ding on the side of my MBA...
So small not visible to the eyes but when u touch the left side of the screen,
you will feel the ding...
From my experience, no. I got my PBs and MBPs serviced with dings on them, it was only once I got the side grill on a PB pushed in quite a bit that they refused service.
Small nicks should be fine. I've seen at Genius Bar people bringing BEATEN UP MBPs and most of times Apple is OK with it unless the machine shows signs of drop or crash. Aluminum is a quite soft material especially when it's thin, small impressions and nicks can happen quite easily.