There's an interesting experiment on rolling resistance related to inflation here: http://www.metrompg.com/posts/tire-p...resistance.htm Basically, for the two cars and their 44 psi max sidewall tires, best rolling distances occurred at max sidewall or a few psi higher. YMMV.
I think Darren at MetroMPG.com has the right answer in that link. The best way to find the point of lowest rolling resistance VS tire pressure is to duplicate his experiment on your own car. Every car is different, not all tires behave the same, so try it and see for yourself.
Thanks Sentra SE-R for finding that link, great info!