My general rule of thumb is go with the max stable clock you can achieve on stock voltage, but if you must use more voltage stick with 40mV or less, or you'll reach the outside of your thermal envelope too easily and your OC won't matter (unless you're using WC).
There's an area of "resistance" that you'll notice when OCing where the additional amount of power is incongruous to the amount of performance you get (ex. it takes an additional 20mv just to stabilize another 10MHz), your sweet spot is right before that. With the 980Ti, in my experience, that's somewhere between 1430-1470 MHz