Most OCers consider a 10-15mhz OC to be very safe/low risk. As someone else mentioned if the frequency is out of range, the monitor will just reject it and revert back to working settings. With the NVIDIA control panel it'll revert to prior settings after ~15s if you don't click accept.
I've been OCing my monitors from 60 to 75 for a long time with zero issues. But as always, OCing is at your own risk, there's always a chance of breaking something no matter how low it might be.
If you're okay with that, OC away. If not, don't bother. But that goes for literally any overclocking, CPU/GPU etc.