A field that reserves no width because it is never rendered is a silent authoring trap, not a safe no-op

If an authoring surface exposes a property, an author will eventually use it and expect it to appear; a schema field that is accepted but silently dropped fails invisibly, which is worse than a validation error. Fingering numbers do have a real horizontal-spacing convention once implemented (they sit above/below the note and rarely add width, but can when doubled digits or a fingering change appears on a chord).

Gould, Behind Bars (fingering placement is generally vertical, so low horizontal-spacing priority once implemented).

This rule can be decided from pitches, durations and voices — no layout required. A validator can check it before anything is drawn.