The default configs for postgres are set for a fairly small memory usage profile (*), which is fine if that's what you need (e.g. tiny vm or something that makes it a huge production to raise things like max shm size), but if you have sufficient ram, you can crank a hell of a lot more performance out of the engine by making the configs less conservative. This page is a good start: http://wiki.postgresql.org/wiki/Tuning_Your_PostgreSQL_Server
Not that it's a priori *wrong* to run with the defaults, it'll still work just fine, but once you start having significant traffic or complicated queries you'll be happier if it more fully uses the system resources available.
(*) It's been a good while since I last had to take a pg instance from stock and tune it, but I very vaguely recall the default settings were on the order of a eight megabytes of ram usage.