We found that pre-paid rates aren't always charged right away, but this depends on the hotel itself.

The overnight charges we found could be charged and show up in statements right away, sometimes we'd been charged 1 week or 3 days before our stays and again sometimes we'd been charged at check-in our check-out.

In theory however such pre-paid rates should be universally charged immediately after booking our rooms/stays with Hilton or any other hotel/chain for that matter, but it's up to the individual hotels to do so, but when charged it shows usually up on our statements.

Cheers & Safe Travels. ^