It won't break your monitor, it's pretty much like cpu overclocking, you can up the speed but you have no options for anything else, if it cant run a certain hz either nvidia drivers will tell you that its not compatible or it'll do the standard 15s count down while all you see is a black screen.
It could potentially lower the life of the monitor, it's doing more than it was initially rated for after all.
Nvidia claimed it was safe to do when they added the option, if it wasn't I doubt they would have put it in there.
