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IME different people prefer different approaches. Some say gear for average cadence across the course, others say gear for the easy bits and others say gear for the hard bits. Personally (in my very limited experience) I find gearing slightly to the hard side of average cadence works best for me, but I guess it's also relatively dependent on the course... most of my fixed TTs have been on the H10/17 variants which doesn't have any major descents/climbs.
Didn't I read "gear for the fastest part of the course" somewhere?