Lithium Ion Voltage vs. Charge Status
4.2V – 100% 4.1V – 87% 4.0V – 75% 3.9V – 55% 3.8V – 30% 3.5V – 0%
Somewhere in the 3.9V or slightly below area would be ideal for storage. Just don't overdo it. I believe AW batteries generally ship about 40% charge, or you can just discharge them in a light or other device, since that is easy and safe.