good to see the notation..... I would think it depends on the type of rocks for adding or subtracting weight to the tank. Lava rock or other light rocks would decrease weigh. But I would think tufa (sp) & heavy/dense rocks would add weight.
The only way this would really apply is if the rock was super dense... however all live rock isn't which is why it is valued for its very porous surfaces(though there are differences in density amongst the different aquaria rock, I dont think you will find a case where it would be a substantial difference.)
A small very dense rock could add substantial weight to the tank(though we don't use them)
A very large rock with low to proportionate weight would displace more water leading to less water in the tank(less water weight) thus the tank would weigh less overall than compared to the same poundage rock in a few very small compacted dense rock... The smaller rock would take up less volume allowing the tank to hold more water, so the tank would weigh more overall.
I don't think you will find a suitable rock that will displace more water than it weighs as it would float and not sink... so it is very safe to say the rock we use weighs more than it displaces. and though it can weigh more than it's equivalent in water weight its safe to round it at 8-9lbs of live rock for every gallon of salt water +/- a slight percentage for large surfaced or denser rock.