If the console has any Internet settings and you don't block the connection with Nintendo's servers, system updates are always automatically downloaded; this is a feature that cannot be disabled.
With regards to updating, there is usually no issue but on firmware 11.0, there is an update for the cartridge slot. This update is irreversible and prevents cartridges from being using on any firmware below 11.0. Not updating your cartridge slot can be relevant for a future exploit that requires a lower firmware and an un-updated cartridge slot. If you care about this, there is a way to prevent the update via nogc. What nogc does is "break" your cartridge slot because if the cartridge slot is not working then it cannot be updated. However it is also fine if you don't care but remember that you cannot undo the cartridge slot update. So in essence you have 2 options: either update the cartridge slot and never be able to use carts on firmware versions below 11.0 or irreversibly update the cartridge slot in order to use cartridges on firmware versions on 11.0 and higher.
Thank you Draxzelex for such a detailed and quick explanation, much appreciated!