TBH, I've got an impression that Manson murders, even if featured in the film, serve as a background at best. It's not the focus of the story. So I never expected some detailed portrayal. Even in Kill Bill, which deals with purely fictional characters and features plenty of violence, QT refrained from showing a pregnant woman being beaten (if we don't count highly stylized animated sequence).
I also think QT in general is aware what needs to be taken seriously and what can be a joke subject in his films. What's tasteless and what isn't. Unlike some other directors.