We use django and some other stuff that has very decent support for postgres, and in my experience postgres updates rarely have breaking changes that require much if any changes for us. This also keeps us from being "trapped" on an old version.

I reiterate from all my systems management - do not update to a .0 release of anything unless there is a critical security update and you're forced to.

EDIT: Oh directly for the question - eh depends. Every 2 to 3 max, and usually after it's tested by others more.

And new projects generally use whatever is newest unless it's a .0 release.