The battery is constantly being balanced. There is no need to worry about this. You want to keep the battery near 50% constantly for best longevity, but in reality, just don't keep it at 100% for longer periods of time and you're fine.
It is always being constantly balanced, but there is an issue wherein if you never bring a battery pack toward the extreme ends of its cycle range it can throw off your SoC measurements. Li-ion state of charge is generally done with coulomb counting, which tends to drift over time, so it gets recentred based on voltage and other factors that are less reliable general measures of SoC but not effected by drift. These are most unambiguous near the ends of the SoC range, so the occasional deep cycle tends to improve SoC estimation accuracy significantly.
Li-ion discharge voltage curves are very flat outside of the endpoints, and consequently the SoC-related differences in voltage are swamped by effects such as temperature, aging, etc on voltage.
(Caveat: Given that Tesla battery management is a lot more advanced than that which you find in general consumer electronics, I imagine that they have a lot of other tricks up their sleeve for helping make SoC measurements more precise. They could subject random cells to deeper cycles (Model 3's "giant motherboard"-style BMS should allow for this), they could measure internal resistance changes, and probably a number of other things.)
