As Altis has indicated, Li-Ion cells store better at a 40% charge rather than at a 100% charge.
I decided that I would keep my back up battery at about 50%. That gives me some emergency run time should my main battery run out, and is close to the optimum storage charge. I think it is a good compromise...
The self discharge rate for Li-Ion cells runs around 1% per month. I have a Li-Ion powered screwdriver that states if you store it full charged, in 18 months you will still have 80% of the capacity left. I think this is pretty accurate.
The Battery University information on batteries aging from the moment they are manufactured is true, but somewhat out of date. The problem with the early cells was the materials they used to seal them. The early cells had a tendency to "leak" past the seals and had a useful life of only about 3 years. Improvements in seal design has improved the longevity of these cells, but you should plan to replace you batteries every 5-7 years, even if you don't use them. I just replaced the battery in my computer after 4 years of use. It is kept at 100% full charge, but I was careful to limit the full discharge cycles.