Can't you do it every time someone votes? The scores aren't dependent on any other items other than their own votes, so each time it gets a new vote, you rescore it and it will drop or go up on the front page accordingly.
Sure, but with a system that is active (like HN), there would be enough votes coming in frequently enough to handle the ranking updates and account for time increases without needing to hit the system every 5 minutes.
Also, if only updated periodically, the front page would "skip" so to speak every X minutes (exaggerated changes in rank), whereas re-scoring on each vote would allow smoother changes in rank.
