Advantage of doing a clean Install of Mountain Lion?

Installing cleanly gives you the advantage that you will know if an error is due to the "as-shipped" condition of the software before you add your data and applications to the mix. The installation of a new OS is the primary time when people notice corruption of a filesystem or other hardware type errors since a machine can often keep running well with some little errors. But relinking all the kernel extensions and drivers and knowing that your accessories and applications will all take to a major new OS is a lot of change. Spending a little extra time up front can help isolate issues if you have them.

By running the migration later, you have more control over things and can also choose to hand migrate anything first that you don't trust the tools to automate.

You are able to fresh install Mountain Lion following this tutorial provided by http://itsalltech.com/2012/07/16/how-to-clean-install-os-x-mountain-lion/

Follow this tutorial in order to clean install OS X Mountain Lion.

NOTE: A clean install of OS X Mountain Lion will delete all data on your hard drive. Be sure to backup all of your files before continuing, and make a list of the apps you'd like to re-install once you have Mountain Lion running.

Step 1: First things first, you'll need to purchase OS X Mountain Lion from the Mac App Store; it'll cost $19.99. Once the installer is downloaded (it'll take a while depending on your Internet connection, 4 GB+ download), don't run it.

Step 2: Head over to your Applications folder. There should be a new app called "Install OS X Mountain Lion." Right-click on it and click "Show Package Contents." Navigate to Contents>SharedSupport and find a file called "InstallESD.dmg." Copy it to your desktop.

Step 3: At this point, it's time to decide whether you'd like to create a bootable installer on an external hard drive/flash drive, or a DVD. We recommend going the external hard drive/flash drive method since it'll be significantly quicker, and that's what we'll show you in this tutorial. You'll need at least an 8 GB flash drive and/or an empty partition on an external hard drive of the same capacity. Plug the external hard drive/flash drive into your Mac now.

NOTE: The following step involves deleting all files from your external hard drive/flash drive. Be sure to backup any important files before continuing.

Step 4: Launch Disk Utility (located in Applications>Utilities) and select the drive you'll be using on the left. Click on the "Restore" tab and in the "Source" field, use the "InstallESD.dmg" that you copied to the desktop earlier. In the "Destination" field, make sure that your external hard drive/flash drive is selected. Click "Restore" and allow the process to take place.

Step 5: Once the process finishes, keep your external hard drive/flash drive plugged into your Mac. Restart your Mac and once you hear the startup tone, hold down the "Option" key. Once the menu appears, navigate to your external hard drive/flash drive using the left and right arrow keys and select it by pressing the "Return" key.

Step 6: Once the installer loads, navigate to Disk Utility and erase your hard drive.

Step 7: Head back into the installer and follow the instructions to install OS X Mountain Lion.

However, if they are brand-new Macs that have files you don't mind keeping, I'd probably just install it the customary way through the Mac App Store. Clean installing is longer, more involved, and perhaps not even necessary. However, it is useful for those who wish to remove all files from their previous install with a fresh start. Hope this helps.