As a representative at Schwalbe, the official line is follow the guidelines on the sidewall of the tire. As a cyclist, I highly recommend reading about tire "Width and Pressure" on Sheldon Brown's website (RIP Sheldon).

If you go to Sheldon Brown's website, make sure you have some time to kill.... a lot of time (especially if you've never been to the site before). No man or woman or other beast has ever put so much good information in one place for cyclists... and for life.