Heh, funny. I recently implemented a countdown for a teleprompting app and that's exactly what I ended up doing to make the countdown "feel right".
The countdown in question doesn't display fractions of a second so it would immediately switch from "5 seconds left" to "4 seconds left" which just doesn't feel right. Adding 0.5s solved the issue.
If you're counting up, round down. If you're counting down, round up. A human expects the count to finish at precisely the moment we get to the last number in the sequence (zero, for counting down). Do a count in your head to see what I mean.
Apple chose a compromise by rounding to nearest, for it to "feel good", but you lose the ability to exactly predict when the timer ends as a human. Typical Apple.
The countdown in question doesn't display fractions of a second so it would immediately switch from "5 seconds left" to "4 seconds left" which just doesn't feel right. Adding 0.5s solved the issue.