Do I need to fill every unused rack position?

No. Leaving some rack positions unused is perfectly acceptable, and spare capacity is often useful.

Blanking plates can improve appearance, prevent accidental access through unused positions and, in some equipment layouts, help guide airflow rather than letting air take an easier route around the equipment. They’re particularly useful where controlled front-to-back airflow matters.

But an unused rack position isn’t a structural defect. You don’t need to fill every gap merely because it’s there.