I am not an expert and there are those in this thread that know more about this than me, but I have my 580's OC to 900mhz. What I have learned is the following:
1. Just cause the slider goes all the way for voltage, you should not max it unless there is a real need to do so.
2. You need to determine what speed you want from your OC. Ideally, you raise the core 20-25mhz at a time, run some tests and if all goes well you can increase some more. When you hit artifacts or it crashes, it could use more voltage.
3. Add voltage as only needed to achieve what you want from your OC. If your goal lets say is 900mhz, research your card to see what others are achieving on air cooling, which I assume is what you are using to cool the card.
4. Test, test and monitor your temps are full load to ensure that you are not going to cook the card.
Hope this helps.