I have used the 4.8V and 6.0V Hydrimax NimH for years with no issues ever. I charge them overnight in the plane with the wall charger from my old Futaba 7CAP radio. As long as I use them a few times each month they are fine but If they lay around unused for several months they will self discharge and die.
I monitor the voltage before I leave for the field, before I fly, and maybe even after I fly.