I concur, book for six. It's always easy to remove a person than it is to add someone at a later date. As long as someone honors the ADR, you won't be charged the no show fee for the people that don't make it. The only places where you'll probably want to remove him from the reservation if he can't go is at the pre-paid locations (e.g., CRT).
You can try showing up with an extra person, but there is always the off chance that they won't be able to seat the person. It's very uncommon, but not impossible, that the restaurant will be at capacity (fire code) and unable to accommodate an extra person. It's also possible that your party was assigned to a table that seats an odd-number of people, so adding someone will mean being assigned to a new table and it might increase you wait time. I can't speak for 5-tops specifically, but I know a few venues that have 3-tops, so parties of three aren't always seated at a four top. It's possible that the same issue exists for any odd numbered parties.
