I honestly don't believe letting the vehicle sit @ 100% is determinetal to the battery. We have tons of Li-ion batteries around in so many devices. Hitting 100% and staying that way isn't optimal but degredation is minmal at best (how many of yall charge your li-ion power tool batteries almost to full to leave them sitting until the next weekend? Everyone just charges to full). Now, the longer the battery sits @ 100% the worse it is for the battery. That I can agree. But I am thinking of order of magnitude different than a few hours (if a few hours @ 100% destroyed the battery it would have been scrapped for a different chemistry - that is just not feasible, ever). Knowing a bit about battery chemistry would leave me to believe that is right.
Now leaving the car @ 100% for a week ALL the time and only driving it once a week...eh, i can see some noticable degredation happening after awhile. Leave it a long time @ 100% for a couple weeks or more (even if its just once) - that probably will hurt the battery some.
Li-ion chemistry is fairly forgiving. No memory loss, quick charge and discharge. Li-ion batteries degrade for three reasons:
1. Time. The second they leave the line they start to degrade evetually the battery will no longer hold a charge, its just inherit in the structure of the battery. There is no way to change this. The battery will have a max set life the second it leaves the factory.
2. Heat. Heat is killer for Li-ions. Thats why the DC chargers stop. If you DC charge as much as I have, the % charge varies quite a bit from 80% - 100%. Its the heat that shuts the system down (to prevent battery damage). In the winter, the DC charger would want to go all the way to 95%+, in the summer, the second that sucker hits 80% it shuts off. This is also why the manual stats "don't plug in unless you have used x %). Li-ion batteries have no quals with "topping off", in fact, under use, they LOVE it. Its the heat generated by the resistence at the top end that is hurtful. As the battery charges, once you hit the top end, its slower and generates more heat byproduct. Its just nissan making sure that the battery doesn't get any heat damage. If you do this once it awhile, I wouldn't worry about it. But, if you find yourself doing it every day...not good.
3. Storage capacity. Have you read treatment of Li-ion batteries you purchase wholesale? Everyone comes with a care of battery storage. ALL say the same thing. If storing for multiple months (some say a month or more - but its always months) keep the battery at 50-60% for optimal life. That % is the sweet spot for Li-ion. Notice the time length. Arguing that leaving the car @ 100% over night is bad and your battery will die in 5 years is ludicrious. It takes a long time for the battery to sit at a high capacity for damage to be done. Also, never leave the car fully discharged, that will brick your battery, plug in as soon as possible!
If you notice all these issues (#2 and #3) talk about abudnant of over exposure and such. The damage done is all exponential. So the difference from a few hours to a few days is minutely small. But get to a week or so its start climbing that curve. The damage still occurs even on the short time frames, but its so small its hardly noticable. If you are shooting for the longest lasting battery record in the guiness book, then by all means follow everything to the T - always 80%, never store at 100%, never top off, don't DC charge, etc. You could probably squeeze maybe a couple of more years out of it before you reach the Time guage, maybe good for 11, 12, hell 14 years! But, lets face it. Few of us are going to try and shoot for a 14 year Leaf. Hell many won't carry it beyond 3 years and get something else. I wouldn't worry about battery degredation unless you hit the extreme times (i.e. leaving it for multiple weeks) or are shooting the car lasting you over 10 years.
My advice: If you can do 80% comfortably, then do it, every bit helps! Don't freat if you can't. The car is still built for 100% capacity and should last you plenty of years, more than you probably will keep it. And don't leave it at full for more than week untouched (I would say 2 weeks). And don't leave your car at 0%! ever!