I like to have all my sites actually in the "Sites" folder just for logic of naming conventions. Under the MAMP Pro scenario, it really doesn't matter, as you can create the virtual directory in any folder on your machine that you desire. But from the standpoint of having an easy label, the "Sites" folder make sense and you could backup the entire Sites folder when needed to ensure all your sites have redundancy. The only other advantage to using the Sites folder is that if you had to use Apache/MySQL that is built in on the mac... I believe that the Sites folder would be the default root of that built in server.