Comment 3 for bug 414997

The image will still "just work" if it is run in a different region without the locale and sources.list being changed to reflect that region.

The image will likely break if the user has made explicit changes which are then overwritten.

I say trust the user to know what they are doing if they have modified these settings, and don't wipe them out.

My proposed approach above lets the image reflect the best-guess rules if the user has not spoken, but lets the user's changes take precedence if they exist.