The cardinal rule with charging NiMh batteries is to get a charger that has an independent charging circuit for each battery. Do not assume that the charger that came with the batteries is a good one. It probably isn't.
MaHa has a series of reasonably priced chargers that charge each battery independently. You want a pulse charger that measures the voltage of each cell independently and turns off the juice to each cell when it is fully charged. This will make a HUGE difference in the lifespan of your batteries.
If you don't use such a charger then you risk over-charging and damaging some of your cells, or in the best case, under-charging the other cells in the charger if the charger happens to measure a cell that is more charged than the others. Over-charging is very bad for NiMh cells.