When most chargers turn on their green light, they are charging in 4.2V CV (constant voltage) mode and the charging current has dropped below a threshold. The charger will just sit there at 4.2V and its up to the cell and its circuits to charge more or not. The cells should take care of themselves if they are of quality. If you are using junk cells -- YRMV, have fire insurance.
Note that the moment the green light pops on, the cell is still charging a tiny amount that will slowly drop to zero, it's just below some setpoint like 50 mA. You can squeeze a tiny bit more in after the light goes green, but not enough that you'd notice in use.