Lucky is right. Break-in is needed for irregularities of metal to metal surfaces to wear down to a more uniform surface. Surface finishes on machines parts are deliberately created to let mating parts polish one another. Also machined parts have an acceptable +/- tolerance. Sometimes the max tolerance of one part fitting into a minimum tolerance of another part can make for a dangerously tight fit. Gentle break in allows them to wear down without causing them to inadvertently weld or seize together from thermal expansion.
When it comes to timing belts, it makes zero difference. Run it!
