For optimum battery health and life span, it's actually better to only charge or store them up to 60%. I believe that's around 3.9V, but you're losing almost half of your battery discharge. Also why laptops using "battery health" mode will only charge to 60% and keep it there. The reason they say 80% is because it's a decent middle ground between 100 and 60. You can expect about 3x the recharge cycles if continuously charged to 80%, and about 8x the recharge cycles if only charged to 60%.
In regards to the discharge limit, it's likely best not to bottom the battery out unless you're OCD and recharge them asap. That 20% or even 10% will give you a little buffer if you forget to recharge them or let them sit a while, it won't ruin the battery. There are also some batteries that have a lower end discharge range down to 2.5 to help maintain that buffer.
Here, I dug this up because I wasn't positive if what I remembered was completely accurate. It's close:
Continuous full discharges will also gradually increase the internal resistance of the battery. I missed this on the first read through.
This is a really good read if you wanna know more.
BU-808: How to Prolong Lithium-based Batteries
