Hi Andrew,
I've read allot on lipo storage on RC Groups.com. The general consensus is around 50% but anywhere around 3.8V per cell seems to be fine. If you are not going to be using your batteries for even a few days, most of the battery experts (especially Everyday flyer on RCG) feel they should be taken down to storage levels. I now only recharge the night before I plan on flying or even that morning. If I don't use a battery that day I take it back to storage mode. That method seems to work well for me as my batteries have stayed strong. If I'm not going to use a battery for at least a few weeks I put them in double freezer bags and store them in my secondary refrigerator. I also let them come back up to room temp (24 hours), before I charge them. This may require a little pre-planning for those that do not have the ability to charge multiple batteries at fast rates. I can charge about 5 batteries every 20 minutes from storage levels.
William