IMO, buy the retail boxes. at least there, you will have the cd key to verify purchase right then and there. with online purchases, there have been problems in the past. also, you get physical discs in the event that something goes wrong. i guess that helps.

lastly, make one (or two) copies of your WoW folder and place it somewhere. this will be your launch for your slaves. its great for itemization of addons (if you want your main to run addons you dont want your slaves to run) and also keeps your settings separate. also, makes it easier to switch to single play.