Normally, this printer centers paper very, very well. In fact, I suspect it uses optical sensing of the paper edge. However, you must make sure that you select the correct *printer* (e.g., Epson 3800) and paper size. On my system (Photoshop in OS-X), if somehow I have a generic printer selected, it will print fine anyway but there will be a minor error in centering when using the rear feeder.
Good luck.