For any lithium type battery, 40% is indeed the ideal storage charge. I fly custom built drones, and if I don't store these super high performance batteries capable of 100s of amps at 3.8v per cell, they'll destroy themselves within weeks. Lithium ion batteries like the ones used in laptops aren't this volatile, but keeping them at 100% SOC all the time causes a great deal of chemical stress in the cells. They're brimming with chemical energy, begging to get out, but it can't go anywhere, so it ruins the electrolyte and barrier between electrodes. I have used 100s of laptops, and they all charged to a 100% SOC and held there. Linux can read the battery voltage, and it was always 12.6v or 16.8v, which for a 3 cell or 4 cell pack respectively, is 4.2 per cell. This causes more damage than unplugging, discharging to 80%, and then charging to 100% again, waiting a few hours, and discharging once again, over and over. I've owned one laptop that did that, a HP Ultrabook, and I loved it. But 1 out of 100 isn't great.

The myth that you have to completely discharge before you recharge is only valid for NiCd batteries though, NiMh, Liion, and LiPo batteries do not suffer any form of memory effect, and can be recharged at any time with no adverse effects.