In "standard" multiverse theory, each universes' delta from the adjacent universe is likely so minute as to not be noticeable (IIRC, the quanta being the time it takes light to travel a Planck Length for each particle), so as long as the "time travel" backwards is to a reasonably adjacent universe, you theoretically wouldn't travel to a universe where it rains donuts (see The Simpsons).
That said, barring a multiverse theory, the grandfather paradox is probably in effect. For some good reading on the unintended consequences of time travel, read http://www.tor.com/stories/2011/08/wikihistory
But would your travel into the past into a separate universe be to an adjacent universe? In the multiverse model, there are allowances for other universes that do not operate with our laws of thermodynamics, quantum mechanics, gravity, etc...you might travel to another universe and completely disintegrate into sub-atomic particles instantly.
Furthermore, even if you could choose a separate universe to travel to, which did operate with our very particular set of physical and chemical laws, how would you get there? Through a wormhole? I doubt you'd get out alive without completely irradiating yourself. And then of course you'd have to harness the power of a neutron star to open the wormhole. And even if you did open the wormhole, you'd have to survive the trip, and have the wormhole remain stable so you can get back home - both on your usual universe, and the one you travel to. Otherwise, you face the staggering odds of trying to find your one true universe in an infinite sea of possible universes (many of which I'm sure would be absolutely deadly to humans as we know them).
Finally, if you could travel to another universe safely, how would you really go back in time? You probably can't. You could theoretically travel to another universe who's timeline has not progressed as fast as your current one in our conventional use of time. And perhaps the only difference between your universe and the one you travel to was that in 1834 at 8:35 am on July 24th in Little Rock Arkansas in a small school, a butterfuly flapped its wings. When you cross from your universe to that one, the version of that universe you experience may feel like 1984, even though that universe is as old as yours. In that case, you may recognize much of that universe (potentially). However, you can't go back into time and affect the past as you know it - you would only be able to affect some other version of the past that potentially some other version of "you" experienced, if in fact that other version of "you" ever came to be in that other universe.
Why do we want to go back into the past? For most, it would be to tell a former version of ourselves to do something we did not do, in order to avoid regret. The paradox of time travel into our own timeline prevents us from doing just that. So in application, there's no real advantage to traveling into the past if it's at all possible. We can't affect our present by going into a past that we've experienced.
We can go into the future to "see what things will be like someday". Maybe not as far as we'd like, but far enough. That said, traveling into the future is wrought with problems. Likely, if you could travel the speed of light and survive, and come back to earth in 400 earth years from now, you'd likely die quite quickly from some bacteria or virus your physiology hadn't adapted to. Furthermore, even if you didn't die, you probably wouldn't be able to communicate as languages will change significantly enough, so you'd feel isolated.
