Are you buying "cured" or "uncured?" A small addition of properly cured live rock shouldn't have any effect on your levels. Just look it over and remove any dead sponge that may still be on it. I have added 10 pounds at a time with no change in any of my levels.
Do a smell test on the rock. If it smells bad then something on it or in it is dead and decaying.
If you have a small bare tank, you can also put it in there for a short time with a powerhead and regular lighting and test those levels for about two weeks. Fill the tank with water and adjust your salinity at least 24 hours before you receive (buy) the rock. If there is no problem in there, there won't be a problem in your show tank.