I have what might seem an odd first step suggestion for grand regulating: Pre-lift wire. This is a strategy to end up with no underlifted strings.
When a piano is newly strung, the steel wire, being stiff, takes rounded curves, not corners, as it passes bearing points. Each time the wire is pitch-raised it refreshes these curves a little, particularly at agraffes and Capo D’Astro bars. A conscientious technician will settle the wire and mate strings with hammers, a process well-known to us all. Touch hammer to unison strings, pluck the strings, and if any ring, lift the others with a stringing hook until all are equally muted.