I've had my Leaf for a week. Leased it for 2 years on Easter Sunday. My commute is 41 miles round trip to/from work. My nearest QuickCharger to home is a 10 mile round trip (I need to find one on the way to/from work) making it about 52 miles for a commute. Since I don't know what my day is going to be, I swing by on the way home and set the charger to 90% and go get stuff for dinner inside the store. I come out and head out when it gets between 80-90%. Repeat daily. I don't own a lv2 home charger (don't plan on buying one) and have never used the lv1 charger after making sure it worked on day 1. Total cost to drive my almost 500 miles I've put on so far? $0.00. Just gotta love those free QuickChargers!
To get back to the OPs question, LifePO4 batteries don't "like" either extreme of 100% or 0%. At 100%, they can swell, heat up, leak, burst into flames, and other fun stuff. At 0% they can reverse polarity and fry your whole pack. As somebody else mentioned, rechargeable battery life is measured in "cycles". To test this, they charge the battery up to 99% and discharge it down to 1%. Repeat. Remember that "life" is an average of the tested group minus a percentage of margin. So user A may get a zillion cycles while user B gets only a jillion. My guess is that Nissan has set their chargers to max out at about 98% of true 100% battery capacity and to shut down the car around 2% to prevent users from trashing their packs and creating warranty calls. I'm also assuming that the battery manufacturer has an added margin built into their specs. There are a small portion of end-users that are extreme idiots, hence all of the amusing warning labels on everything. To further support this, I was offered a "battery degradation warranty" when I leased (I pointed out that the pack was covered for the term of the lease under the normal warranty and declined.) Nissan would not offer this unless they had set in place some safeguards on their packs. As such, I intend to charge my Leaf daily to 80-90% at a QuickCharger, take it in for the 7,500 mile check-ups (which I understand includes a pack balancing charge and is included in my lease), and enjoy the surprised look on the BMW owners when I floor it. I also base this assumption on what I know the computer industry has done for years on laptop batteries and their charging cycle.