It depends on the station. SEA (at least the Concourse D and N checkpoint) requires all footwear to be removed. I have seen open-toed shoes with soles measured in microns be removed and screened.
My shoes evidently don't set off the detector (since I walked right through LHR with them on), but due to their design (hiking boots) I know they meet the TSA profile so I humor them and always take them off and run them through.