Is it true that only the pre-cat o2 sensors have to be replaced at 100k miles? I heard the post-cat o2 sensors aren't really that important.

BMWs made before 1995 have 1 or 2 o2 sensors mounted in front of the cat. converter. BMWs 1996+ have ones in front and behind the CC. They all serve a purpose and need to be replaced at one point, or your car will surge like mad and let you know. O2 sensors have a life expectancy of 60-100k miles.