It's about assessing the risk, following a hierarchy of control and then implementing those controls.
Generally would look at isolating the area, then use a spotter when their are maneuvering risks, risk of unauthorised persons in proximity to the MEWP or overhead services.
I would be cautious of implementing a blanket rule, as some situations a spotter could increase the risk, if they are unskilled, distracted or in a potential dropped object zone.