Well, depends on the rock you choose. How long has the tank been running? What is the water volume?
Larger tanks may not even notice the addition of a few pieces of rock, smaller tanks may. The longer the tank has been setup, the less effects you will notice to other parts of the tank.
If you use wet(live) rock from your lfs it can come with hitchhikers that you may or not want. You could dry it, to kill off anything organic.
Or use dry rock, however there is much discussion regarding dry rock leeching phosphates into the water. I see it a ton with Marco rocks, but this ends up as the rock looking like a fuzzy algae magnet in the short term. Long term, the equation will balance and it will be back to normal.
But, if your phosphates are already high, certainly something to be aware of. If that’s the case just don’t add a ton of rock at once.
The other option for dry rock, is to assemble it all how you want it, and drop it into a bin of saltwater and seed it with bacteria for a few weeks to a few months depending on how patient you are. The longer the better. Leech the phosphates into the bin rather than your system. And very little algae(or none) will grow because there is no light.