I agree with all the points mentioned so far, and as both Mark Davidson and sdanelson have mentioned radio coverage I just wanted to slightly expand on this as there are a couple of areas:
Signal strength - generally you want to use the minimum signal strength possible in a particular area, so an attacker outside your side can't gain access, but this can leave you open to an Evil Twin attack if a malicious access point copies your SSID and uses a much higher signal strength.
A solution is to think of your propagation paths - locating your access points around the outside of your site with antennae configured to be directional into your site will help a lot as you can increase the access point signal strength (so looking stronger to the clients) without propagating your signal so far outside the side.
A simpler, but more effective solution I have seen (which may be overkill for you - I have seen it used in a very sensitive establishment) is metal clad walls and ceiling with mesh in the windows - a wireless site survey of this site picked up zero RF leakage!
Security through Obscurity - in this case hiding SSID beacons - gives you a false sense of security. Your Access Point will still broadcast SSID, just not in the beacon frames. Many ordinary users will still be able to connect as drivers for many platforms will still identify the SSID, and all attackers will be able to find you as they normally would - try any of the tools on the Russix LiveCD and they will work quite happily with SSID broadcast disabled.