it doesnt look like any of the gtx760's have heatsinks applied to the back side at all so you are likely fine.
if you plan on overclocking the card and have already bought a bunch of memory heatsinks to stick on the front side... then it certainly isnt going to hurt putting them on the backside.
looking at the heatsink configuration of a stock card though.... since there is nothing on the back i would say its optional. (at least on the backside).