It will also depend on the type of live rock you will be adding. If completely uncured "raw" rock, it would be best added just after the tank is set up. That way you can both cycle the tank and try to preserve as much life as possible by controlling the water environment.
If using cured live rock, there will still be some die off to help fuel the cycle but not enough to prolong the cycle time the uncured will and could actually shorten the cycle time somewhat. Quite honestly it is best either type used if you add as much as desired or afforded from the start. Each time you add rock down the road, it will spur a small spike in the water quality. If there are animals added to the tank later on, any new rock should be re-cured seperately prior to placing in the tank.