Its not worth it, most of the different brand thermal pastes only have like a few degrees differences in temps at best, I'd only change the past after the card ages, I still have a GTX 780ti reference model I installed into my aunts PC to play some of the little games she plays and I never changed the paste on that card since it was new, I've only ever taken the top off to clean the dust out, but never unscrewed the cooler from the card.
The card still runs around 80C at full load, the same as it always has since new.
If you ever think you need to change paste int he future, just load up a synthetic benchmark or a heavy game and let it run and watch temps, I try to keep below 80C, but its not uncommon to see over 80C, I'd say around 90C you'll probably want to figure out why its running hot, either needing to be cleaned from dust, fan speeds, or even old dried paste, it'll be a year or more before swapping thermal paste would be recommended.