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.
Rule of thumb, you want OFDM enabled on all your SSIDs. This will get rid of DSSS encapsulation (802.11b protocol) that slow down your entire network. Also, your management rate will bump up to 6 Mbps. Start there.
After your network has ran several days with no complaints, bump it up a bit. I would say to 12 Mbps and let it run for several days. Go around and ask about any Wi-Fi issues, if non exist, then try 24 Mbps.
Going above 12 is where it can get interesting as Brian pointed out. Depending on your deployment, you might run into roaming issues/coverage issues. There is no one right answer here. Play with it and be patient to observe any changes.