The key word for wipings is 'random'. The daleks master plan was wiped in the 1960s but mission to the unknown survived until 1974. Go figure.
Yep. This thread led me to work through the wiping dates given in Richard Molesworth's excellent (but now outdated, in that you need to read his DWM pieces to see what he's turned up since) and actually scribbled it all down by date.
It does generally make sense, if you assume that some episodes got wiped later because they got missed first time round (which is supported by cases of episodes getting authorised for wiping in one year, and then again a bit later when someone notices that they haven't been wiped).
A general picture is:
Mid 1967: Seasons one to three (Hartnell is no longer the Doctor, most of them are more than two years old, and Enterprises have made new stored field copies). Strays get wiped over the next two years (the strays have generally survived because both drama and Enterprise have to Ok a wiping, and sometimes a form has got missed/got lost).
Autumn 1969: Seasons four to six (Troughton is no longer the Doctor, there's no plans to repeat black and white Who before the new colour version starts in January, Enterprises have their copies). Strays also get wiped as it's noticed that they've survived.
After 1969: Odd surviving tapes go bit by bit: An Unearthly Child ep 1 (both versions), Mission to the Unknown, War Machines, Macra Terror, Fury from the Deep, Invasion and War Games. Macra Terror seem to have survived as long as it did because the Wipe-Ok form had the wrong tape numbers on it.
But there are atill some oddities: some episodes are listed for retention after they've been wiped, some are wiped just after being listed for retention (Evil of the Daleks), and Highlanders goes astonishingly early (as if the normal retention paperwork just got missed).