Can't say I'm a big fan of flash forwards, it's such a delicate balancing act of not giving away too much information and basically the writing becomes spoilers.
It's not a spoiler if the writers want you to know what happens in the future. "Spoiler" means something that spoils the intended experience of the story. If the intended experience is for you to know the end first and then gradually learn how things led to it, then the spoiler would be not knowing the future. For instance, telling either Citizen Kane or Sunset Boulevard in linear chronological order, with the main character's death at the end instead of the beginning, would betray the intent of the narrative.