Try running the Pepper app. What you'll see if you go too high with insufficient voltage is that dies start stalling, and the temperature will drop.
I got these from gateway last night, so I'm pilfering them but attribution.

This is a board at 950MHz, with different voltage setpoints.
930mV


940mV


950mV


970mV


You can see the temperatures drop constantly at 930mV; the chip is being starved for voltage. It's much better at 940mV, and at 950mV three of the dies are running fine. At 970mV they're all running good.
With the python script gateway posted you can even adjust the dies individually. For the above case, you could do
./hftool.py -w 0:950@950,1:950@950,2:950@950,3:970@950
That would set the first 3 dies to 950mV and the last one to 970mV

Note that cgminer overwrites the frequency value (the number after the @) with it's setting, unless you use --hfa-hash-clock 1
That can be useful if you're really trying to push the board. You might end up with a die that just can't do 950MHz even at higher voltages, but the other three can get up to 1GHz. If that's the case, you can set the defaults for that board individually and run some of the dies faster than the others.