A beam's vertical placement relative to the staff should read cleanly — clear of staff lines, not ambiguously resting on one

Gould's guidance (and general engraving practice) is that a beam should visually clear the staff lines with enough margin that it doesn't appear to be sitting on, straddling, or merging with a staff line — the beam should read as clearly above or below the affected noteheads.

A beam for a group of notes near the middle of the staff is drawn at a Y position that lands directly on a staff line, making it hard to distinguish the beam from the line at a glance.

Gould, Behind Bars, ch. Beaming — beam placement relative to the staff; Ross, Art of Music Engraving — similar clearance guidance for legibility.

This rule is about where marks land on the page, so it can only be judged after engraving. Collision and spacing faults live here.