I think it really depends on what you're willing to pay. You see some really "good" deals on live rock, but normally I think you get what you pay for. You can get uncured, "cured", or really and truly cured - all with varying degrees of hitchikers.
I bought all my rock online, as the stuff I was seeing locally didn't really do much for me. I figured I couldn't do any worse online, sight unseen. After researching around, I gave premiumaquatics.com a call. I didn't order online, but instead called and talked to someone there, described what I was looking for size and weight-wise, what size my tank was, and asked them to pick out about 90 lbs of rock they thought would do the job. (I have a 46 gallon, and figured I wanted a little extra in case I had to break some pieces up, or didn't like a piece they sent.) They came through with flying colors! Great rock with shapes that worked perfectly for me. It wasn't the cheapest rock in the world, but it was far better than anything I saw locally - and I have about 6+ fairly good LFSs within an hour of me. It was already cured (for real!) and was shipped airport to airport with me picking it up at my local airport within 18 hours of when it was packaged. I had a very slight cycle and no major die off.
I guess my point is you can order "online", but still talk to a real live human and get a feel for their inventory, what kind of shapes and sizes they have, and how "live" it really is. You won't be able to do this with all online suppliers, as many of them don't even see the stock themselves. But there are some great folks out there with great customer service.