Wi-Fi Tip: Channel Utilization Best Practice

For best performance of latency-sensitive applications over wireless, such as voice, plan to keep RF channel utilization under 50%. This is not a hard-and-fast rule, but a best practice for most networks supporting data and voice.

Check out these resources for more info:
Cisco Voice over WiFi Troubleshooting Checklist
Wireless LAN Professionals Podcast #10 - Designing for Voice

-Andrew