If you got a Nicad, then you should discharge it before. Nimh's do not need this type of treatment, but I would recommend cycling your NiMh once in a while. I maybe wrong, but topping a partially used battery, NiCad or NiMH, without discharging should not be a problem.
Or get a NiMH and simply don't think a about it.
L'Expression De Mes Sentiments, Les Meilleurs