I started a wiki page to document this information . There's an old page  that probably should be revisited and merged with this one.
While this isn't settled, I'd like to ask people that file bugs to add a tag referring to the image version where the issue was found, e.g. bug found at image 27, tag the bug "r27". Also please tag "regression" in case you found one. This helps us to spot problems that should be fixed before promoting an image to stable.
Ursinha (Ursula Junque)
Ubuntu - I am because we are
Linux user #289453 - Ubuntu user #31144