I`ve recently bought a brand new iPhone 7 32Gb from Apple Store in Shibuya, Tokyo and it has no gap at the top, bottom or right side of the frame (I especially looked for it, even opened box right in the store - I am always checking my new iPhones for gaps, dust in camera lenses, etc.). But yesterday I`ve noticed a SLIGHT gap on the left side. It is not even wide enough to stick a regular sheet of paper into it, but it is there (maybe because of lightning in Apple Store, I haven`t noticed it while checking for the first time).
I`ve decided to not to go for an exchange, because the next model I will get may have bigger gaps or other issues.
Furthermore, starting from 7 and up iPhone models have dust and water-proof gasket along the whole frame, so if even this gap collects some dust, it will not get to the internals of the phone. Cameras are also has their own gaskets and secured with plates, so it has 2 lines of defense against dust!
But, if gap would be bigger, I`d definitely go for an exchange.
Also, shame on apple QC!