Dropouts are made from malleable steel for just this reason, so they can bend a reasonable amount without breaking and be realigned. Will it break next time? Possibly. The biggest factors are the sharpness and degree of the bend. As these factors increase, so does the proabability of forming a crease, which causes a stress riser. The risk of future failure druring subsequent cold setting increases proportionally with the severity of the crease.

Dropouts can withstand mutiple small and gradual bends with out failure. I've serviced bicycles where the derailleur hanger has been realigned probably half a dozen times. Kids with X-Mart bicycles are constantly dropping their bicycles on the drive side, which bends the hangers and throws off the indexing. You tell them not to do it, but the bicycle comes back again with the same issue. It typically takes several visits, mounting costs and, most importantly, the increasingly irrate parent, for the message to sink in. This is one of the most common bicycle repairs.

The bottom line is that, having seen the actual bend, your mechanic is the best qualified to assess the risk of failure during a subsequent cold set. If you're fond of the frame and concerned of the risk, you could always install a derailleur guard, which would take the brunt of future accidental impacts and substantially decrease the risk to the hanger.