Early Ni-Cd (Nickel-Cadmium) cells had a reputation for losing capacity if they were not discharged fully during each cycle of use. This was dubbed 'the memory effect', i.e. the cell remembered that you didn't use it to the full and changed accordingly.
Modern developments in Ni-Cd technology have largely eliminated the memory effect, but the myth of its existence persists. In fact, more damage is done to cells by people trying to avoid the effect than by people who ignore it altogether; the reason being that batteries are likely to fail permanently if discharged significantly below 1V per cell. This is exactly what happens if you leave equipment on until the batteries are completely flat.
It is also inadvisable to leave Ni-Cds unused for long periods, because they will self-discharge slowly and eventually drop below the point of no return, so take them out occasionally and recharge them.
If you are superstitious about the memory effect, use an electronic battery discharger, i.e. a device that takes the battery down to 1V per cell and then stops drawing current.