With a mechanical mod the voltage isn't around 3.7 at all for most of your vaping time, hey I usually charge my batteries when they come down to 3.8v.
As an example, may daily work mod is my Reo Grand. I leave home with a freshly charged 18650 at 4.2 volts, and when I come home 7-8 hours later the battery usually is at 3.9. So the majority of vaping is done between 3.9-4.2volts, and definitely not all over the map.

Today I brought a second mod with me, but used the Grand most of the time, the battery is now 4.05v still.
With smaller batteries like 18490 and 18350's you'll need to change more often of course, but the Panasonic 18650's I use have amazing performance.