The clain is that a Li-Ion battery should be stored at near 50% charge in a cool place. It will last for many years like that.
Me, I charge them to 100% and keep it handy in case I need it.
I just bought two 1500Mahr batteries AND a charger for about $8. While I doubt they are true 1500MaHr batteries, I'm not going to get carried away with caring for them.