You don't need to overdo the charging / discharging stuff, but, especially when they get a bit older, NiCads have a habit of 'remembering' how much you discharged them before and, if you didn't completely discharge them, you might find they won't charge fully.

There are plenty of threads on the subject here in RCUniverse that will give you a better informed answer than me, but, as a rule of thumb, I tend to cycle the batteries once every ten chargings.

-David C.