I prefer the Slider's theory of time travel. Traveling into the past just sends you to a dimension that looks exactly like the past. So completely screwing up that world would not affect your present or create a paradox where you cease to exist because your father never met your mother.
Well if we go by the premise of the Abrams films, doesn't matter what you do, observation, voyeurism, tinkering with history, whatever you do the future you return to won't be the one you came from, so the message to potential Time Travelers is, make sure your life insurance is paid up, because you won't be back.
Unless you were going back with invulnerable shields and a weapon that never runs out of ammo you really wouldn't like medieval times. The only safe kind of time travel would be where all you can do is observe. A window to view the past is ok, a doorway to interact with it is a really bad idea on too many levels.