I think that this relies on a faulty premise--that modern knowledge would actually be an asset in earlier times. I think the argument could be made in the last 200 years or so, but after that modern knowledge would probably be a severe disadvantage.
Case in point, hygene. We are used to it here to protect us form disease and well discomfort. However, back then the lack of hygene would not only be distatasteful, but potentially deadly (read bacteria, etc). Notice as well that past about 100 years the food would probably kill you.
Notice as well that basic concepts of human interaction have changed dramtically. Just showing up at a particular time in a particular place speaking a strage dialect would probably get you killed PDQ.
Also one would be quite unprepared to do the basic chores of life. Think making ones own clothes, butchering meat, procuring clean water, etc...and even if you could manage, the time it took would leave little time to take over the world.