You can only leave your batteries on charge overnight if you have a proper charger that ends the charge cycle once it reaches optimum voltage. Optimum voltage is 4.20 at the highest, and contrary to popular oppinion, 4.22 - 4.24 should not be considered "just fine".
The chargers that do this and are easily available are the Pila Charger and more recently the much more cost friendly Xtar WP2 II. While they charge your batteries to 4.16-4.19 , they allow you to leave them overnight/unattended for long periods of times, even days.
If you have a Trustifre TR-001 , forget about leaving them overnight and take them off the charger once the light turns green.
Storage voltage for batteries used for vaping purposes is generally 3,8-3,85v , so if you don't use your batteries for a while, you can have them sitting at that voltage. There is no reason to "top up" batteries once a day, if you use a couple in rotation that means that you've charged them recently anyway, and they hold the voltage. One thing wich is usefull for the battery life over time is to charge it once it's empty, not 3 days later. Also not overdischarging it helps aswell, most people use their 18650 between 4.2 volts and 3.5 volts , but even as low as 3.3 doesnt dramatically reduce total charge cycles ( 3,3 will give a lousy vape anyway on a mechanical mod, so even without a meter generally people swap batteries for a fresh one long before the voltage is significantly low).
Imageshack - img20120618130201.jpg bonus link to my almost 3yr old silver bullet receiving a beauty treatment
