Associating a purchase order to a visit does affect the job costing inside SingleOps, and it will appear in the Expenses table on the job's page once the purchase order has been approved.
Associating a purchase order to a job does not affect the job costing inside SingleOps. It will, instead, appear on the Purchase Order table on the jobs page.
It is worth noting that the two appear the same from the QuickBook's perspective after they've synced. It is only within SingleOps that they differ.