It actually doesn't matter when you pass the boost, It's more of a matter of oppurtunity. As long as potential taunt/encore users have been removed to get a successful smashpass, and counters to your smashpass receiver have been weakened enough for it to sweep, then your're good to go. Sometimes it's actually better to pass the boost as early as possible, since even if you fail (like due to a critical hit), the opponent's team will probably be weakened enough for the rest of your team to finish it off.
In this case, i knew suicune would be able to take one hit, but it can't OHKO without a calm mind boost. So it was imperative that I try to sweep before Suicune sets up, since suicune is a pretty big threat to my team overall. (and I'm not sure how beat it without completely altering the team, though Flygon is pretty removable)