Is it better to keep LiOn battery charge on the lows or the highs for battery lifespan?
For storage purposes, a Li-ion battery should be charged to 60-70% capacity. This reduces the damage done to the electrode material over time while still allowing enough charge so that it will hopefully not be completely discharged due to self-discharge when you get around to using it.

For charge cycle purposes, a Li-ion battery should be charged during its constant voltage cycle with a voltage about 150mV less than the voltage specified in the datasheet. This will result in only about 85% to 90% runtime versus full voltage, but an increase charge cycle count of 20% to 40%.

Of course, if the battery is in a device then all this is moot since the device manufacturer decides what "full" and "empty" mean for the battery; you're only responsible for connecting the power supply to the device.