Here's a link to the NICS FAQ, thought you might find it helpful.
Federal Bureau of Investigation - National Instant Criminal Background Check System - Guide for Obtaining your FBI Identification Record
When we were originally interviewed for our initial FFL, the ATF person assigned to us, strongly advised us to not release firearms until the response comes back vs. the stated requirement of the 3 business days wait on a delay response.
It has happened that 3 days is not long enough for them to come back with a deny and then there is the rather ugly process of reporting this has occurred.
It's a tough position as an FFL to be in, just comply with letter of law or go the extra level of assurance.
Even in the video training for FFL's which can be found on the ATF website, they use a cartoon of an owner letting someone know that it's store policy to wait for the actual response vs the standard 3 day wait for a response and transfer at that time.
Thankfully this situation doesn't happen all that often in our experience. It is frustrating for all involved to be sure.