Typically power resistors are designed to operate without a heatsink (or have one built-in). If the manufacturer recommended use of a heatsink, it would be covered in the datasheet.

That said, they can get quite hot, so care should be taken to keep temperature-sensitive components away from a power resistor that is dissipating a lot of heat. Additionally you may want to ensure that the resistor is mounted with an air gap *below* it as well (not touching the PCB) to promote air circulation on all sides.

A surface mount power resistor with a pad for heatsink may require a copper pour/heatsink or a physical heat sink of some kind attached (see the datasheet for the component in question).