Regarding a battery tender--I doubt that it would hurt anything, although, in my opinion, it is not needed. The LEAF's DC-DC converter comes on periodically (every 5 days on 2011 and 2012, not sure about the timing on later models) to charge the 12-volt battery while the LEAF is parked. I parked my 2011 numerous times (as much as 3 weeks at a time) at the airport or my office without incident until I forgot to unplug the Bluetooth interface from the OBDII diagnostic connector and left the Android device running LEAF Spy Pro on the seat. That one time, I came back to a dead 12-volt battery after 6 days. I now double-check that I have unplugged the Bluetooth device from the diagnostic port when I park for more than a couple days.
Regarding charge level--I always charged Silver 679 to 100% because I needed the range every day. I will do the same once I get my new 2015. Based upon battery capacity reports on MNL, I don't think my battery deterioration was any worse that those who charged to 80%. Charge to 100% if you need the range, otherwise charge to a lower level. As Tony said, the most important things for battery life in our climate are to keep the battery as cool as possible and avoid having the car sit at a high state of charge level. Whenever I parked long-term, my battery was at about 60% SOC and there was never any noticeable loss of charge during storage.
If you want to track detailed information about your LEAF and it's battery, I highly recommend the LEAF Spy Pro application with a Bluetooth interface to the OBDII port. Just remember to unplug it for long-term storage.
Gerry