I haven't heard anything about it toughening up the gums, but I've read in a hundred different places that it causes a choking hazard because it numbs the back of the throat. I've also read on various blogs that it doesn't really work anyway because all their drooling washes it away pretty fast.
Have you tried teething tablets? They worked well for us when I didn't want to keep giving them Tylenol. They sell them at Walmart.