I like jo77's golden rule. I also tend to think that if your guitar is properly set up and you can't tune the open strings correctly by ear, you may want to practice some ear training. I don't think it's an issue of perfect pitch or rare talent or anything like that, it's just training.

Notice that a symphony orchestra tunes entirely by ear. True, the oboe may begin from some standard reference before giving pitch to the concert master (or is it the other way round?) but each member of the orchestra then fine tunes entirely by ear, adjusting each instrument according to jo77's golden rule.

But hey, that's just me. :-)

- Mark