*get ready to be bricked alive*
Thing is, I find it a lot easier to believe that going back in time, regardless the reason, does create a different, alternative timeline. Which is why I did have a bit of trouble accepting when Scotty and McCoy gave the transparent aluminium formula to that engineer in The Voyage Home. It would have made more sense to try and find the company which actually created the formula and maybe provide them with the knowledge a bit before it actually happened.
The way I've always seen time-travel is a like in Back to the Future, i.e. if you change, even accidentally, something about your past or that of your ancestors, it's certain that the reality in which you existed will be different, even compromising your own existence. For instance, if I had gone back in time and had prevented my parents from meeting each other, I wouldn't be here today, would I?
That said, I do wonder what else Nero's attack on the Kelvin changed in terms of the Federation, its policies, etc, etc...after all, if they knew it was a Romulan ship, I find it hard to accept that the Federation wouldn't ask Romulus for an explanation, at the very least.