[asylumdown]

How do you define 'new tank syndrome'?

Are you talking about a spike in ammonia? That's only possible if the rocks you're putting in have lots of dead stuff on them. Marco rocks have very little on them that could cause an ammonia spike, but some of the dry fiji rock that I've seen looks like it might.

That can be mitigated by letting them sit in salt water for a week or so, but if it was a small amount of rock relative to the system, and the system was mature, I'm not sure I'd even bother doing that. A mature system has many different pathways for dealing with ammonia, and it's capacity is pretty elastic. It's why you don't see ammonia spikes every time you add a new fish to a mature system.

Anyway I'm pretty relaxed about most of this stuff, so other people might disagree, and it is technically possible for other things your don't want to leach out of the rock like phosphates, or to cause a spike in nitrates, so it's never bad advice to say let it cure in a bucket of salt water with a power head for a week or two, but that's about as far as I'd go. If you did that, you're pretty much guaranteed for it's measurable effect on your system, other than increasing real estate, to be nil.

ETA: the only other effect of adding dry dead rock to a system that I've read about is some sort of temporary impact on alkalinity. Not sure of the chemistry behind it or if it would be a big deal if it actually happened, but that too could be mitigated by soaking the rocks in salt water for a week or two first.