Should I leave empty U between switches purely for cooling?

Not automatically.

Some equipment is designed to sit directly above or below other rack equipment. Other devices rely on particular ventilation openings or may benefit from additional clearance. What matters is how the equipment actually moves air.

If a switch draws air through the front and exhausts it through the rear, leaving a ceremonious 1U gap above it may achieve almost nothing. Carry that idea far enough and you could turn a 12U cabinet into six switches separated by six beautifully ventilated empty spaces while the actual airflow remains essentially unchanged.

Conversely, packing equipment tightly around genuine ventilation openings can cause a real problem.

Follow the equipment’s airflow requirements rather than leaving gaps as a cooling ritual.