Ok, there are instances where I have seen here that ich may come in on LR [believe it or not]. During the 3rd stage of the parasites’ live cycle, it does “fall off” fish and does end up falling on sand and LR. Now, of course, this is only a problem if your LR has shared water with infected fish. To be absolutely sure, you could go ahead and hypo your current fish, and when you add the new LR, it will be going into a fishless tank too for a period of time [long enough to get rid of ich if it has any on it], thus eliminating the possibility of contamination.
If live rock tanks share filtration with fish tanks in fish stores.......then, well, you know the rest. You can ask you LFS about this, to see what he says.