Just sitting vertically is OK in truth but the issue comes along when you pull the lever. For some reason it is impossible for friends or visitors not to pull a lever as they walk by.
That is true - people love grabbing the brakes. Our bikes, however, are hung high enough for people not to grab them. Bikes in the repair area are hung the same. Yes, if you grab the lever while the bike is vertical, its not a good idea. But like I said, I store many bikes at home vertically and I have nothing to complain about.
Is it the best way to store bikes? Of course not. Is it space efficient with high ceilings and limited area? Yes.