AFAIK it's somewhat of a "damned if you do and damned if you don't"-situation. Even though there's still people walking around who claim that even LiIo/LiPo batteries suffer from the dreaded "memory effect" of olden days, this is simply not true.
The main factor in LiIo/LiPo wear (given decent charging circuits) is the number of charge cycles performed -- with the bonus that partial charges actually count as such against the total number of cycles typical for such a beast (usually somewhere between 500 and 1000, depending on a variety of parameters).
Obviously keeping the battery on the shelf in a cool and dry place at 60% charge will give you the longest lifespan, but then again it wouldn't be of much use to you
Every time the battery is kept for any length of time at a charge above or below that (or at warmer temperatures), its chemistry will suffer, albeit not as bad as some people will want to make you believe.
My experiences with LiIo/LiPo powered devices in the last 5 or so years more or less confirm this. So as long as the devices are not getting too warm while on mains power (laptop computers are really nasty buggers when it comes to that), I usually keep them there and return them to it for recharge when no longer needed. The results for me (including the Z, extending to cell phones, digital cameras and the like) have been most satisfying.
Best regards,
Chris.