What will crack glass is the temperature differential more than the actual temperature (at least at the temperatures most people will encounter). The difference between boiling water/ice water and boiling water/salted ice water is minor. I can tell you from experiment that a borosilicate toy will go from a 300+ degree oven to ice water no problem, so boiling -> salted ice water is fine. In fact, I had to drag out a torch before my test toy cracked.