Yea, i'm curious too.
I've had to move health packs to the opposite side of the room/into a corner just because i feel it's more appropriate/out of the way of cover. I mean... the small health and ammo in the hut by the BLU spawn is just poking out into the doorway... there's no way that's a legit item placement.
Plus a lot of the ramps are off the grid...
Pretty much any addition of a single medium sized prop can drasticly change the dynamics of gameplay in the local vicinity. "Common sense" also differs significantly between each individual so i'm not inclined to accept that as an answer to such a query, particularly since what constitutes a "significant layout change" is open to interpretation. But i also feel it can't be answered without directly showing a mediator an image and asking for a seal of approval. That's a lot of work for such a seemingly minor thing that is inevitably going to accur across all the entries.