I tend to agree - part of the fun of open tunings is surely that it helps you to think more fluidly about the music, and not get hung up with tonic, sub dominant, dominant, relative minor cliches.

Still you go about it any way you want - there are no rules. I would recommend martin Simpsons excellent instructional DVD on open tunings. And what really got me into open tunings was meeting a great guitarist, namely Ken Nicol (Steeleye Span) at a seminar at Fylde folk festival. he showed me ways of playing DADGAD, that were a revelation. I had previously thought it was just for these busy bee Irish rhythm players, and traddy balladeers.

A good guitarist who can explain himself will show you a lot quicker than trawling through the websites and instruction books.