Apple won't do anything unless battery health goes below 80%. I've been running between 87% and 90% for the past few months on my '11 Air. It's definitely heading down, but so what? Real world usage is the only thing that should concern you. If you only get 2 hours from your Air, definitely take it in, but if you're running between 5 and 7 hours, you should be fine.
It is perfectly normal if your battery health (maximum capacity) is more or less than 100%, even when brand new, or if it fluctuates up or down over time. For further details, read the CHECKING STATUS AND HEALTH section of the following link. Apple usually won't do anything unless your battery health is below 80%.
The link below should answer most, if not all, of your battery/charging questions. If you haven't already done so, I highly recommend you take the time to read it.