I would really love to see an answer to this question come direct from S&B. I have seen literature about care of Li-ion batteries, and the good practice of a Goldilocks approach - not too low and not too full. Sure, this is science and it's pretty clearly true.
However, as
@Eatrocks is saying, the device, as many do, probably has some amount of battery management built in to the circuitry. Idk more about that, but my imagination tells me they might do some things like; unit shutting off before battery is really fully depleted, or; unit slows and/or stops charging before fully charged, even if showing 'full' bars.
I was told a year ago by a S&B rep that Crafty is not a 'smart' device, which I distinguish from say, a Pax2, which says right in the packaging that it is a smart device. Pax doesn't care how much time it spends on charger, while Crafty/Mighty apparently do. So I'm just saying we can't necessarily second-guess the device.
All this said, do I coddle my Mighty and set a timer to take it on/off charger? Nope! I do as I please and have a great-working Mighty for almost 9 months now. I see that lately if bars are low it takes a little longer to get to temp., but not very much longer (yet). I assume that one day it will need new batteries, if the rest of unit itself lasts so long. At that point I may have to hone my soldering skills!