The belts are finely machined to very small tolerances, and things are tightened to specific foot-pound tolerances to make sure they sit exactly where they need to. So no, nothing slips.

I just went through a belt change with my brother not long ago. The tolerances you're dealing with are measured in thousandths of inches. Something that wasn't installed quite straight can cause almost imperceptible wobble that can destroy things under load.

Still, every day shop tools will help you get the precision you need.