It wont really hurt the cells to discharge them at a slower rate, it will just take longer. It is best however to discharge at a consistent rate, which in most cases is 30 amps because that is what most battery matchers match the cells at. Likewise, it is best to always charge them at the same rate. Cells like being "trained" to always do the same thing. Definitely discharge them though, no matter what the voltage is. Even with the NiMh cells, they will last much longer if you discharge them.