As theorized over at Slashfilm, it's entirely possible that Tarantino will alter the course of history and have Tate survive. Why? Well, we know Tarantino has a penchant for punishing real-life villains at the expense of historical accuracy, as he violently offed Hitler and other high-ranking Nazis at the climax of Inglourious Basterds. Could he be planning a similar end for Manson and his family?
Notably, Tate's sister Debra was vehemently opposed to the movie until she read the script. She then claimed that the movie "is not what people would expect it to be when you combine the Tarantino and Manson names." Which could perhaps indicate that Tarantino's Tate won't meet a violent end.
We'll go one step further down the escalator of rampant speculation: What if Tarantino has Bruce Lee kill Manson? Lee (played by Mike Moh) shows up briefly in the trailer, pointedly bragging that his hands are "registered as lethal weapons." And in what could potentially serve as foreshadowing, Pitt goes on to explain the rules of manslaughter. Then, as further evidence that this movie is a full-blown fantasy, they spar and Pitt somehow isn't unconscious within four seconds.