There's no right or wrong answer here. Traditionally, the more densely packed you deployments of clients and APs are, the higher you would set these rates. If you're needing clients to be supported at the far reaching edges of your APs RF cells, the lower you'd want these set.
Basically, the BSS Min Rate is the lowest data rate your APs will use for basic communication/traffic. So if you were to pick 12 Mbps and your client needed to use 6 Mbps, it would be forced to find another AP to join.
The Mgmt Tx Rate is the rate at which mgmt frames (Beacons, Probe Reponse, etc) are sent. Again, the higher the rate selected, the shorter range that they can be demodulated by clients meaning the smaller RF cell.