Actually, time travel is not exactly the same in all episodes. For example, in Assignment Earth, it's almost described as fatalism, since the outcome was set, and they basically couldn't fail. A single timeline, where incursions actually are a part of the events. It's like in The Time Traveler's Wife. Illogical, to say the least, since time traveling has to start somewhere.
Moving on, in The Future Guy-episodes, we have a single timeline which in fact can be altered. It's pretty much the same in TBOBW, First Contact and Year of Hell.
In Paralles, which isn't really a time travel episode, but relevant non the less, we have multiple universes, each for one possible outcome of actions. It would support the idea that it might exist a prime and an alternate universe, as in Star Trek XI.
I prefer the Future Guy one, but saying that Star Trek XI time travel is unprecedented isn't really true.