My two cents: Interference between wifi and zigbee is practically a non issue these days. For example, at CES over the last few years, people have been unable to get their wifi devices (old but it demonstrates the point: http://blogs.reuters.com/mediafile/2011/01/06/ces-please-turn-off-your-phones-and-your-wi-fi/) to connect at all, but the zigbee products were working great. Since then, more wifi devices have shifted to newer revisions and 802.11G, N, etc. actually make it easier for zb networks to share the frequency band compared to 802.11B (faster data rate so more quiet time for zb devices to talk).
Of all the things to look at for making the zb networks/devices more reliable right now, I would put channel selection at the bottom.