You should never put your hand into a tank with a dangerous wild animal until you are totally comfortable with risks. That said, stingrays have the barbs just as a defense mechanism. The only time I've ever seen one of my stingrays actually whip their tails was during mating; the younger female was being flipped and mounted by the larger male, and she was protesting rather violently. The male was incredibly agile in dodging her blows, and his angle of attack prevented her from actually landing any blows. She has since given birth (1 stillborn, her first pregnancy) and he is at it again.
We hand feed our stingrays; been doing it for over a year now and have never observed anything but peaceful behavior from these beautiful little monsters.