I consider batteries a “consumable”. 3.7-3.8, or on high end chargers, you can use a “storage” setting that will discharge a battery down to that voltage level. I used it for LONG TERM storage, in double sealed containers stored in my refrigerator. 5 years later, my Sony VTC5’s, were fine.
Not sure why anyone would be concerned about the discharge voltage of batteries that one is using in rotation. (Other then a mech mod with NO cutoff or protection, 2.5 volts is as low as most lithium batteries should be discharged) Most modern electronic devices that use lithium batteries have a safety LOW VOLTAGE CUTOFF. Usually around 3.3 volts, and when the device reaches that voltage, it shuts down. Properly made “REGULATED” devices should deliver the voltage till the cutoff.
Standard lithium charge is 4.2 volts, and the process repeats. I get the most out of my batteries, using them in the simple way they were designed for. The spent batteries, I put in a battery case, until the 4th of my five 18650’s, in my rotation gets spent. Now, I have four batteries to charge at once in my 4-bay charger.
If one is mostly concerned with long term battery life, which even with anal care is not going to last that much longer, then ignore my post. Most of the devices we use, need a fully 4.2 volt full charge, down to the devices cutoff voltage to get the best vape. At $4-$5/each, I just buy new batteries when it’s time.