As 'Hooked on Fenix' said, you can top them off any time. Two things to add:
1. This is only correct with an 'intelligent' charger, that detects when the batteries are charged. With timed charger you might overcharge them and this is detrimental to their health.
2. From time to time you should go through a full discharge / charge cycle. Best done with a good charger that offers this function. This prevents the voltage suppress effect.