I can slide mine just a tiny bit (half a mm to a mm), but it takes actually trying to move it with my hands to do it.
Just having it sitting in my lap moving around or on a desk moving it around and it doesn't move. It doesn't make any noise as if it's clanking around loosely. And it'll be a rarity to get one that's going to be 100% perfectly flush.
Now if it actually sags vertically (like some of the MB's have shown) or is loose enough that it actually makes noise and shifts quite freely, then I would exchange it.
But like I said, I can slide mine a little, but it's very snug, and therefore nothing to worry about.