My definition of dead rock is rock that once had bacteria and critters in it but have been left out of water for such a long period of time everything has been killed off. Yes, there will be die off from the store to the tank. However, it shouldn't be so significant that it'll cause what uncured rock would cause. A little jump is inevitable with live rock due to the die off, but there shouldn't be any reason for an ammonia spike so great that it would be a cause for concern. For example, if I have a tank that is up and running with fish, inverts and everything in it, should I add LR, it shouldn't cause a spike so great that it will kill fish and inverts. It should cause a slight hic-up in the ammonia but nothing that a slight water change can't resolve.
EDIT - Also, I had forgotten to mention that I had also started my tank off with Live Sand to help kick off a cycle, and help seed the dead rock.