Sealed (or flooded) lead acid batteries must be kept well charged. They self-discharge and sustained periods of deep discharge (less than 70%-50% of the capacity remaining) cause the plates to permanently degrade. The exact schedule depends on the brand but they usually need to be topped off at least every 6 months. Ideally they need a small trickle charge, unfortunately common "trickle chargers" actually put out many times too much current for maintaining these batteries and you end up with a slow overcharge overcharge over months or years which actually harms the batteries. But be aware that all lead-acid batteries only work for a limited number of years (5-10) anyways, even if unused but ideally maintained. "Use it or lose it".
Li-ion should NOT be stored full charged but rather at about 40% remaining. But note though that Li-ion should never be completely discharged. This won't happen if you ran it down in a camcorder for example but if you put a resistor across the cells it will. Leakage of charge typically fairly minimal. Also keep in mind these are expensive cells and permanently lose capacity as time goes on. They lose 20%/yr if stored full, that's down to 4% if you store them at 40% charge as instructed.
The point is, don't buy these in bulk, use half and keep half in storage for when the first ones run down. Don't buy them just to store them.
NiCd, NiMH- as far as I know there are no storage issues.