Once upon a time, in the days of 80-column punch cards and no variable-length character encodings, there were databases that could handle fixed-width character fields a bit faster than variable-width. That doesn't apply to Postgres. There is no, none, nada performance advantage to char(n), and you should never use it unless your application data clearly demands a specific field width.
I still often use char(n) a lot, mainly for documentation purposes. If a column will only ever hold exactly a fixed length string (status codes and stuff), it looks better in the datamodel to use char(n) IMHO.
Tim
