If they use the same SSID and channel they will NOT interfere with each other - they should gracefully take turns according to the 802.11 specification.
Only if they use different SSIDs and the same channel (and are in range) will they interfere with each other.
The drawback of using the same channel for six APs would be essentially sharing the bandwidth between all six. A 54Mbps wireless connection provides only 27Mbps of actual throughput, maximum. If you have ten clients and everyone is on the same channel, you're effectively limiting each one to about 2.7Mbps of bandwidth. Where if you put two APs on channel 1, two on channel 6 and two on channel 11, they won't interfere with each other and you effectively triple the available wireless bandwidth. If all you have is a T1 line (1.5Mbps) and they aren't talking to anything but the internet (no file sharing between them or a local server), then having them all on the same channel's not going to make that much difference. I hope that makes sense. Feel free to ask for clarification if it doesn't. :-)
I've not seen any problems with roaming when they're on different channels, by the way. The windows wireless zero configuration (WZC) service doesn't allow you to specify a channel for an SSID profile, anyway.