I've been keeping rays and other fish including catfish together for a number of years and have only seen one stinging incident where a siamese tiger fish went for an item of food that was close to the rays tail and got impailed under the throat. Fortunately the ray was due to shed its stinger and it just came away from the tail and remained imbedded in the tiger in one unbroken piece, all i had to do then was catch the tiger and remove the stinger using a pair of pliers. Both fish still live together in the same tank today some 2 or 3 years later.
While stinging accidents can happen it is a rare occurence and not one that happens often enough to warrent not keeping them together in my opinion.