It would normally be considered at a design stage. For domestic installations no special precautions are needed because most of the heavy loads are usually heating elements which, as true resistances, do not create harmonic currents. The contribution of any distorting loads (e.g. phase-angle controlled dimmers, half-wave rectified electronics etc) is small. Industrial installations might include large loads with specific characteristics that have a significant impact on the overall installation, such as rectified inputs to VFDs that do not have PFC. These would be known in advance and designed for, and in certain cases, loads that could disrupt the power quality for other customers would have to be agreed in advance with the supplier.
From the wiring point of view, a distorted waveform on a single-phase circuit is not going to cause any problems provided Ib<In<Iz as both conductors are protected by the OCPD. On 3-phase installations with zero-sequence harmonic loads (e.g. the 3rd harmonic component of a phase-angle controlled load), the neutral current can exceed any one line current therefore the OCPD may not protect an equal-sized neutral and an increased conductor may be required (or, a reduced conductor might not be acceptable).
