Brett,
Look up the charge/ discharge curves for Li Ion batteries.
The 3.7 V is a nominal voltage representing the middle of the discharge curve. Charge them to 4.2 V per cell and stop using them when they fall to 3.0 V per cell.
There is not much capacity left below 3.3 V.
In use the cell voltage will be mostly anything but 3.7 V.
The advantage of LiFe chemistry is the much more stable cell voltage in use. And of course it’s ruggedness.
73
Ron
VK3AFW