At what currents should NI-MH batteries be charged?
As PlasmaHH commented, for optimal life and full battery capacity the charger must follow manufacturer's specifications. 

Rechargeable cells can be different, normal charge, rapid charging, fast discharge, etc. In case when detailed specifications (like this one) cannot be found, the rule of thumb is to charge NiMH batteries at 0.1C (C=rated capacity). Therefore, for a typical AAA rechargable 1.2V battery with typical capacity of 800 mAh, the charge current should not exceed 80 mA, so the 500mA is a clear overkill, literally. For AA with 2500mA typical capacity the charge should be done at 250mA, but 500mA could be fine (0.2C rate). Unless the battery cell is designated as "rapid charge", when the charge current can be at 1C or more. So, the answer is "it depends", and the mileage can vary.