It is always best to add cured rock to an existing tank. Uncured rock will have dieoff, so if you get uncured, you could set it up in a QT tank or trash can to allow the die off before adding to your main tank. If you cure in a trash can or spare tank, remember to change the water so that the curing process won't kill all the beneficial critters on the rock. Just watch ammonia/nitrite levels as you would when cycling a tank.
John