Hi Dave...that used to be my practice, particularly if it has not been cured, but....
Since the rock I will be getting has been "pre-cured" and there should be minimal die off at this point, the only thing quarantining the will achieve is potentially identify some unwanted hitch hikers. I could experience a small cycle, but right now none of my corals or fish are in my main system, they are all in my quarantine system.
Let's assume the live rock comes in with AEFW, quarantining will not do anything for those suckers and I am not going to soak my live rock in Bayer. What about red bugs? Unlikely they come with the live rock, but I could hit the rock with interceptor, which I am not going to do because I don't want kill off all the micro fauna etc.
How about mantis shrimp? Regardless if I catch him in the quarantine system or my tank, he/she still needs to be caught.
They way I see it, the only reason to quarantine (Or cure the rock again) is because it could cycle. If the rock was coming directly from the ocean to my tank (aka TBS Live rock) then curing the rock really makes sense to me.
Any other reasons to quarantine/cure the rock again that I am missing?