If you start the tank off with live rock and sand immediately and it is truly "cured" live rock, you shouldn't have much of a spike. That's because the bacteria is already in place. Even if that is the case, it is still a good management practice to wait for 6 or 8 weeks before diving in head first into live stock. The tank and water still need to age. Young tanks also have a tendancy to recycle even if something small goes wrong.