I tend to think some methods of time travel create alternate timelines while others don't, though I'm sure some people would be irritated by the inconsistency there.
I think I mentioned in a different thread that after Parallels, at least, you'd think Starfleet Procedures would involve checking quantum signatures anytime a time travel situation occurred, to verify that people were in the timelines they were supposed to be in.
Agreed. I know this isn't canon, but it makes for interesting speculation-
In fact, in the novel Preserver, Picard says that such procedures were indeed put in place. The book also implies that the Mirror Universe came about as a consequence of a Schrödinger's Cat-like scenario (with Zephram Cochrane being the cat in the box) created as a consequence of the Ent-E/Borg visit to Earth. In the book, the memory erasure doesn't take hold-and Cochrane asks himself whether he should tell the Vulcans, ultimately leaving it to chance-the flip of a coin. Heads, and the Prime timeline comes about. Tails, and the MU is born.