Absolutely, the butterfly effect renders all back in time plots fatally flawed. Never mind killing Hitler's grandfather or making sure your parents still meet, your very appearance in an earlier world, even for an instant, would guarantee that before long you and everyone else who would otherwise have been born afterwards would certainly never exist.
I saw a drama based on Steven King's 11/22/63. A guy goes back in time to save JFK from assassination. Spoiler: he saves JFK in the past and when he finds his way to the future the world is destroyed. Oops.
This presumes a variant of time travel that allows changes to make a difference. A many world interpretation would just mean branches (and you'd only see your changes if you manage to find a way to navigate to them); a fully deterministic model would imply you can't change anything and your time travel always had happened exactly the way it would happen, and whatever you think you achieved was consistent with history (wasn't really Hitlers grandfather after all for example); a strongly convergent model would see the effects mostly have relatively minor effects, and when they tip over and have bigger effects, those bigger effects still end up cancelling out over time.
We both don't know how time travel would work if it is possible, and don't know well enough what the odds are of any small change affecting large long time change.
E.g maybe if Hitler didn't come along, someone else would have.
And in fact, exploring those effects is often a major part of back in time plots.
Moorcocks 'Behold the man' for example have someone travel back in time to meet Jesus only to realise that the real Jesus was a mentally disabled man and that since he is the only one that knows the image of present day Jesus, the time traveler himself must always have been the person history knows as Jesus, presenting a deterministic idea of time, where paradoxes would be impossible, because time is in that world in effect just one more spatial dimension our perception moves across.
There are many such models that would allow for some time travel plots to be plausible all the time we have no idea how it actually works.
Of course all of them will probably look quite ridiculous and quaint if we ever figure out the real thing.
