I've ranted about this in the early postings on my "
Life with an Epson 7900" blog.
I use a Mac with OS X 10.6.8. For several years I used a Canon iPF 5000 (also used this with a Windows system before moving to the Mac). With the Canon, if I fed a sheet of any size, and checked the necessary control in the driver or PS plug-in, the image was always perfectly centered on the sheet. This assumes, of course, the image was small enough to fit within the minimum 3mm margins one could set for many media types.
However, with the Epson 7900, with the latest driver and firmware installed, checking the "Center image" checkbox in the print driver is certain to print an image that is not centered. This is true even if the image is considerably smaller than the sheet size, such that there'd be margins of a couple of inches. The only way to print on cut sheets and center the image is to make a print that's NOT centered, measure the offset, and then adjust the top margin by half that amount. Print again with those values, and you'll be close to centered, but even jumping through that hoop will only get you close most of the time.
I called Epson support about this and was told to print on roll paper. The tech was clearly uninterested in pursuing this further. Not exactly helpful.
A good friend with the same printer, run from a Windows 7 machine, has said his prints are centered on cut sheets. We've even tried printing the same size images on the same size sheets; we get very different results. This clearly points to a Mac issue: the OS, the print architecture, or the Epson driver. Or, perhaps there's a problem with Photoshop (I do all my printing from PS). I can't say where, but there are bugs somewhere in that chain.
I do a fair amount of contract printing. I think it looks very unprofessional to provide prints to clients that have weird, non-symmetrical margins. I really wish Apple or Epson or Adobe, whichever (or whatever combination) is responsible for this, would fix it!