I think that such a fundmental part of the mission design as the scan strategy should have been set in 1994 instead of floating around until 2008. The very old choice of no reaction wheels severely limits the current range of options. And for polarization one really wants to scan through a given spot on the sky in many different directions. Planck will scan through a given spot with polarizers in many different orientations but they will be on different detectors. This imposes even harder requirements on the calibration and stability of the detectors. I would have made a different choice back in the 90's but I wasn't asked. See "Scanning and Mapping Strategies for CMB Experiments"