0.9V is the default. When you try to overclock the chips, hardware error rate increases. Raising the power supply voltage helps reducing the error rate somewhat. In the end, you're trying to get the maximum speed with an acceptable error rate, or the maximum effective speed of hashing as reported by your pool. Both raising the supply voltage and overclocking the chips will increase the total power consumption of the chips and reduce their power efficiency, i.e. GH per Watt. It also increases heating and puts more demands on your thermal design. You can measure power consumption by using an ampermeter on the 12V input power supply wire.

If you're optimizing power per GH, you should instead try to lower the chip supply voltage and speed, until you get the best GH/Watt ratio.