The problem with on line live rock is it isn't necessarily very live any more. It is shipped, from where ever it is harvested, by boat to the vendor then shipped to you. It has been out of the ocean for a long time. There will be bacteria on it but a lot of life dies off in the transport process. From a fish store it may not be great like fresh from the ocean but the bacterial colonies should be good and if transported wet there should be minimal die off. There are exceptions but they cost a lot more. Tampa bay saltwater farms live rock which is harvested and shipped airfreight in water. The life is still intact. Some people even find little fish in their shipments. BRS sells live air rock which is harvested somewhere in the pacific I think and flown to the states a couple times a month. This is much fresher live rock as well but not as fresh as Tampa bay SW. More money.
I like to get dry rock and seed with a couple pieces of good live rock. Not as good as straight out of the ocean but cheaper in the long run because dry rock weighs less.