v8.4 and v9.2 (Yes, I know they're archaic and out of support, but there's nothing I can do about it.)
Some of the tables in the database which I manage (but don't design) have btree indexes on CHARACTER VARYING(256) fields. Even when the field values aren't all blank, the length of the data in them is no more than 12 characters.
Yes, the physical table storage for these columns is highly compressed, but what about the btree indexes? Would the indexes be more efficient if the columns were altered down to VARCHAR(12)?