Best practice is to perform a Site Survey, and diagram your necessary coverage area and number of clients. Then you have to locate APs where they can reach all the required areas. Part of a Site Survey is to find out what sources of interference exist (AC/Heating, machinery, Floresent light ballasts, etc), so you do not place APs too near to these.
I suggest implementing ChannelFly over Background Scanning, which will utilize more channels in the 2.4g band, overlapping as little as possible with neighbor APs. I recognize many of your clients only on channel 6.