I'd wager that 99% of time travel movies are just so flawed that you have to shrug and accept the faulty premise. Oh gee, John Connor sent his father back in time to impregnate his mother, only that couldn't have happened since John Connor was never born. It's dumb, but it's time travel, so you accept it.
But there are some stories that work really hard to make time travel not as fragile. I recall Millennium and Primer as being fairly solid (though I haven't seen the former in a long time so please don't shatter my illusion if I forgot something).
And then there are the moments where you say, "Screw it; time travel! In your faces, bitches!" Hitchhiker's Guide to the Galaxy and Doctor Who easily fall in this category. They were already absurd to begin with, so they just break the eggs and make an awesome omelet. Although, Doctor Who does have some rules in place, but their enforceability is highly reliant on how the plot can develop.
Personally, I don't think time travel exists. Well, outside of the usual +1 sec/sec rate of travel. It's an interesting theory, and it makes good for fun stories, but I suspect that this is an immutable fact. Then again, there were people who claimed that reaching the moon was impossible, so you know…