Depends on the battery type.

NiMH should be run down from time to time, say once every 5-10 charges
or so depending upon usage patterns. The good chargers will not
overcharge them, but some cheap chargers can overcharge. If the charger
has a cycling design that first discharges and then fully charges the
batteries, even better.

LiIon works best when topped up and should not be completely discharged.
They also can not be overcharged because of their design, so putting
them on the charger an hour or so before use to make sure they're topped
up is safe and won't hurt them.

You can google for more details, but that's the basics.

   Well, I would watch trying to discharge them.  Few batteries take being
fully discharged and most methods of discharging batteries chance reverse
charging which can be deadly to batteries.

   A good way of adding to the life of your batteries is a quality charger
that will custom charge each battery rather than the cheap ones that tend to
charge only pairs or groups of four at the same time, overcharging some and
undercharging others.