This isn't a Doctor Who post, actually. I read an article on Tor.com about time travel as it is portrayed in movies, and what kinds of various consequences it is shown to cause. They really should have mentioned the Doctor, since they included Bill and Ted, and we all know where they got the phone box idea from - right?
So, time travel. With or without consequences. Or self-consistency, which is another way to say that everything will turn out the same no matter what you do, because history dictates it. (Fixed points in time, anyone?)
Have a read at http://www.tor.com/blogs/2013/11/story-worlds-time-travel-part-1.