Speed deselect is not a recommended mode of operation in the standard FCOM, but it is in boeing issued company tailored ones provided the correct training and SOP around it is used. We do it that way now for over 18 years both in classics and NGs without one single tailstrike. In fact the average tailstrike margin on the -800 is much higher than it is on the airbus fleet.
Especially during flare i do not really see the problem as the autothrust is in flare retard mode below 27' anyway and doesn't try to keep any speed. Above that it will intervent when the speed drops of course.
Anyway, operate according to your company SOPs, but dont switch off your airmanship and common sense, in doubt just switch everything off and fly it manually, allways works best.