12Mb is considered to be the normal minimum setting to use as you can get strangeness with some chipsets especially for their initial connections if you set a higher value.
My son's HP laptop will not make a new association (after a rebuild) if you set the minimum rate at 18Mb or above, after the very first association you can then turn up the minimum bit rate without any issues (so I think HP were doing something stupid with their stack though.)
I dug through every Wi-Fi equipped device that I have access to and other stuff have strange things when setting up a new connection, with Nintendo products being the most horrible so that isn't really an issue for most Meraki installs.
For reference, some won't make an initial connection if band steering is enabled, some insist upon 802.11b speeds being available for the first association, then they are happy running on 802.11g
I only mention this as I am sure other devices suffer from strange behaviour too such some Zebra (Motorola) devices can't connect to a certain competitors APs for example due to DHCP options not being parsed properly, so even though the the spec says things 'should be good' reality says otherwise.