In the case of BTTF1, I have to disagree. Every time Marty comes close to altering the timeline in such a way that he would not be born, he starts to vanish. This means, that there is only one timeline.
That’s a very good point, actually. I always preferred BTTF1 by a wide margin because of seemingly random-ass things that suddenly appear in the sequels (the “chicken” thing, any new character related to Marty in another time is suddenly played by Michael J. Fox, a couple other things), and you pointing that out just adds another example of the dissonance between 1 and 2/3 that I hadn’t noticed before. Huh.