Stock-voltage overclocking is considered the "safest" way to OC.
If you don't have too much voltage and/or too much heat, instability should be your only problem once you push too far.
Once your card "crashes" or starts showing artifacts, just back off the speed by about 20mhz and you should be good.
Make sure you watch your temps. It's unlikely that a stock-voltage OC will overheat your card, but more speed can create a little more heat, even without a voltage bump.