We ask you to keep a few intermediate saves until the results are published, in case we have any query on the progress of your game. 1000 BC would be needed for QSC anyway so we prefer that you keep that plus saves around 10 AD and 500 AD.
It can also be helpful for your own sake to keep saves at critical points in the game such as change of government, era changes, major war events, so that you can go back and take screenshots or retrieve other information for the spoiler posts.
A further point is that it could be dangerous to rely on Autosaves to start a new session after a break. Playing any other game such as a quick personal game or an SG *could* wipe your autosave out. So unless you are playing in a single session, manual saves give better security between sessions.
Disk space is cheap, and you don't have to keep them long, so why not save often? One option is to use the amazing CivAssist2 utility to record a complete archive of your game. At the end, you can then use it to create those cool animated minimaps showing your territorial progress as a movie.