Personal preference, adding it prior to the cycle will just cycle the sand along with the rock, live sand doesn't need to be cycled, but it's certainly going to boost the bacteria and cause a faster cycle this way. Adding it after the rock has cycled isn't going to phase anything either. I'd add it prior to the rock myself, unless you are just tossing the rock in, letting it cycle and then going to worry about aquascaping.
Another train of thought, some people prefer to add their live rock and then put the sand in, the sand built up around the base of the rocks will help support the structure, while sitting the rock directly on top can cause a more unstable structure if a fish were to burrow underneath a ledge. ( I have my rock just sitting on my sandbed, no problems currently because of it)