We set it for manual approaches as well in order to keep manual & automatic approaches as similar as possible & also to have the target speed that we are attempting to fly shown on the ASI.
While your SOP tends to contradict the Boeing 737 FCTM advice I can understand why you prefer to stick to one command speed setting whether for manual or autothrottle.
Does that mean you approach at Vref + five knots for all final approaches (autothrottle engaged or not engaged) regardless of HW comp and gusts? The problem with deviating from manufacturer FCTM recommended practice is that it requires a risk management approach which needs to be cleared by Boeing to cover your backside from legal attack if something should go wrong.